It takes a lot of talent to be a consistent goal-scorer in the NHL. But on any one individual goal you sometimes just need a little bit of luck and a good bounce to go your way.
With the Flyers in an early 2-0 hole against the defending champs, Hartnell attempted a centering pass to Claude Giroux from behind the net. Instead of making its way to the Flyers' captain, the puck bounced off Blackhawks forward Marcus Kruger, then hit Blackhawks goalie Antti Raanta, and finally settled in the back of the net.
In the hockey world, this is referred to as puck luck.