The result of each game : is it mathematical or just random ?

[Tyltyl]

Possession isn't a reliable stat to read the Game. More possession is never equal to more opportunities. Your scoring opportunity is dependent on the meter at the bottom of the screen in a live animation, the animation would never show your team attack if the meter isn't in your favor. This meter is more dependent on your formation and orders than it is on possession. For instance, if you set a defensive/ counter-attack strategy against a stronger opponent even if the opponent has 65% possession it doesn't matter, if the game engine favors your tactic over his own the animation will give you more scoring opportunities. A better stat to keep tabs on a live match is shots on target both for and against, and how many of those shots forced the goalie into making a save

Maybe I wasn't clear enough, but opportunities come exaclty out of that and I partially agree with you except for one thing.
I always play defensive and counter attack and I have a lot of possession in my favor everytime, but when I played against this team which is weaker, he got a lot of bonus possession out of friends watching up to +15% in his favor (at my home with me watching!). It was enough for him to get 55% possession, 7 shots and 4 goals.