The difference is that most times, teams come back because they give a renewed effort while the other team backs off.
Sure there are times when a very close game can suddenly be even due to a lucky goal. But in this game you can be up 5-0, and then for some reason in the third period every little turd from the point magically finds its way behind the goalie. Every deflection goes to the opponent's stick, every legal hit is a penalty against your team. Passes go astray. Shots you were scoring on in the first two periods suddenly go wide or hit every limb on the way to the net. If you play with AI, they will suddenly forget to be in position like they were at the beginning of the game.
And it's not just something that happens against you all the time. I've also been on the receiving end of these situations where my team absolutely sucked the entire time. Then when my opponent goes up 3-0, everything goes my way. Every shot hits the net. Every pass is crisp. My AI act like the 1980's Oilers. And suddenly my opponent who was absolutely destroying me can't hit pass from 5 feet out. Can't hit the net. Every rebound and deflection goes to my players. His goalie becomes a sieve.
It's not fun...