The scores are not predetermined before the game, the best theory I've heard is that the game is updated either twice or four times during play. If it was every 15 mins then your score could well have been decided with 10 mins to go.

The reason I know it's not before the game is that I tank a lot of seasons and many times I wait until I know I can't win, say 6 or 7 nil down, and then I change my formation to a legitimate one for the second half or last 30 minutes, and I'll always pull a few goals back.
This has happened thirty or forty times at least, where my changes have swung the game drastically in my favour, far too often to be coincidental.