You will never find a reason, the game is always right. Wanna know why?

Because there are several factors that are hidden to the opponent. You will never know if your morale kicked in, and was so better that the illegal formation didn't matter. You will never know if your condition was so much better than his, so that no matter how you played, you did better. You will never know his tactics as well. Maybe globally, he was so low on everything, that the "less worse" team won.

That's how the game and the devs troll you.

Also, we will never have the instruments to replay the same game twice with the same identical conditions to understand what actually mattered and decided the match.

On a sidenote, I just played a league game that I won 1-3 after being down 1-0 after the first minute. What I saw was that in the 1st minute (FIRST!!!) I already had 2 shots and my opponent 1. Well, that was a bit much for a "realistic" football game