Home advantage, morale and condition, tactics, training ... lots of factors could have changed.
But (as is usual with these rants) you don't mention the number of shots, or possession rate, number of saves or any other stats that give clues to what happened.
Instead it's always "My team's average rating is better than his!!!!!"

And yes, the game engine IS random. Duh? Other than chess and Diplomacy I can't, offhand, think of a single game that isn't random.