6 NFL teams are still undefeated. This is why.

The NFL prides itself on parity, but right now, it feels a little top heavy. There are only eight teams with winning records through a quarter of the season, and six of them are undefeated. Five of those are 4-0 and the 3-0 Patriots could hit that mark with a win over Dallas on Sunday as well.

Those six undefeated teams -- the Patriots, Bengals and Broncos in the AFC, and the Panthers, Falcons and Packers in the NFC -- look to be in full command and on track for the playoffs. So, how are these teams so dominant? Where are they winning? How are they getting it done? Let's take a look at the stats.

Offensive performance

As far as traditional counting stats go, the Patriots are clearly winning with a dominant, unstoppable offense. Led by Tom Brady's surgical passing attack, New England is leading the NFL in points per game, yards per game, and pass yards per game. Despite playing one less game than nearly every team in the league, the Patriots have the third most points.

Brady's completing 72 percent of his passes and has nine touchdowns with no picks thus far. If he can manage to keep his yards per attempt (8.4) rate constant through the season, it will be his highest mark since the 2011 season when he passed for 5,235 yards and 39 touchdowns. It would even surpass his absurd 2007 season (he hit on 8.4 YPA that year). Oh, and by the way: Brady is on pace for 5,930 yards passing, which would completely destroy the single-season NFL record (5,477).

So, that gives you an idea of what's going on in New England thus far. They are throwing the shit out of the ball.

Meanwhile, the Falcons passing attack has blown the doors off opponents. The Matt Ryan-Julio Jones connection might be the most potent combo in the league. Atlanta is third in the NFL in points per game (34.2) and fifth in total yards and passing yards. This team can move the ball.

Some context into how dominant Jones and Ryan have been through the air: Even with a relative dud last week (four catches for 38 yards), Jones is tied for the lead in NFL receiving yards and is on pace for a line of: 152 catches, 1,912 yards and 16 touchdowns. At this pace, Jones would set the record for catches (currently 143 by Marvin Harrison in 2002) and his pace is just off the all-time record Calvin Johnson set in 2012 for yardage (1,964).

The Bengals have been moving the crap out of the ball as well, both through the air and on the ground. With a stable of excellent and healthy offensive weapons at receiver, tight end and running back, Andy Dalton has flourished. At his current pace, he'd end the season with 4,748 yards, 36 touchdowns and four interceptions on the year. In addition to an excellent passing attack, the Bengals are running well too. Balance has been a big factor for their success in the tough AFC North.

(via Football Outsiders)

My favorite advanced stat is Football Outsiders' DVOA, which measures efficiency on a play-by-play basis. It helps give some context to how effective teams that run less volume-oriented schemes are, in other words.

Through four weeks, DVOA passes the eye test offensively, with the four top teams all in the undefeated club. The Patriots are first in overall offensive DVOA, second in pass DVOA and first in rush offense DVOA. This team can move the ball at will, in other words. The Bengals aren't far behind, and are essentially right there with the Pats -- second in overall offensive DVOA, first in pass DVOA and second in rush DVOA.

Unsurprisingly, the Packers -- who run a slower, low-volume offense (they're the fourth-slowest team in offensive tempo, in fact) -- are still potent from an efficiency per play standpoint. Green Bay is third in offensive DVOA -- third in pass DVOA and sixth in rush DVOA. That's strong balance from a team that is mostly known to be an Aaron Rodgers passing attack.

The Falcons are up there (fourth overall, fifth pass, third rush) in every category as well. The emergence of Devonta Freeman as a viable workhorse back will help Atlanta maintain their offensive tear.

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Defensive performance

The Broncos are clearly winning with their stifling defense, as evidenced by their offensive rankings, both traditional stats and advanced. Denver is holding opponents to 17.2 points per game -- second fewest in the NFL -- and have given up the fewest yards of any team thus far this year. Teams can't pass against their excellent secondary, quarterbacks get hurried by their tireless pass rush and opponents can't run against their front seven either. Overall, Denver might have the best defense in the NFL from a traditional stats point of view.

The Panthers are winning with their defense as well, which is no great surprise -- it's been very good for several years now. More importantly, they've been doing it mostly without Luke Kuechly. Carolina is currently third in the NFL in opponent points per game (17.8), and they do it with a balance between pass and rush defense.

One surprise team on that list might be the Packers, who started out with a stinker in Chicago, but quickly put things back together with the return of B.J. Raji. Green Bay is surrendering the fourth-fewest points per game so far this year, and have had a stingy pass defense thus far.

The advanced stats match up pretty well with the traditional ones. The Broncos are the NFL's best defense per DVOA, and the Panthers aren't far behind at fourth. The Packers are sitting there with the seventh-best defense thus far -- fifth in pass defense but a staggering 31st in rush defense. They have improved on that mark since Week 1, when the Bears put 189 yards on them on the ground, but that will be something to watch going forward.

The Bengals are still sitting in the top 10 in defensive DVOA, so while they may give up a lot of yards, they're still efficient from an opponent points per game point of view.

Both the Patriots and Falcons are middling on defense, and both clearly lean on dominant offenses to win.

What's it mean for the rest of the season?

The Patriots and Falcons are clearly still undefeated because of their dominant passing attacks and efficient run games. They can score points with the best teams in the league and their defenses thus far have been good enough to stave off comeback attempts. They say that defense wins championships, but there are certainly exceptions to this rule if you're talking about a good enough offense.

Gary Kubiak's offense has taken some time to find its footing this year and Peyton Manning isn't the prolific passer he once was. Luckily, Denver's defense is suffocating -- and that buys the Broncos some time to figure things out on offense. I don't know if they can take this thing all the way with a near-league-worst offense -- probably not -- so they should have to get Peyton going or establish some sort of run game.

At the same time, the Panthers are leaning on their defense after losing Kelvin Benjamin prior to the season to an ACL tear. Their offense is still getting the job done, but a highly ranked, stingy defense is their calling card.

Right now, the league's two most balanced teams might be the Bengals and Packers. Both can move the ball efficiently and with explosiveness, and both are strong on the defensive side of the ball. If one side struggles in a game, the other can pick them up.

Finally, it's worth pointing out that there's still a ton of the season left.