i dont believe it's rigged...but its prolly precalculated before the match starts.
I mean, there are millions of matches played every day.....imagine if all the matches are actual simulations and that the outcome of every match can be altered while you watch it. that would take some really good server power to deal with that many real time simulations.
sure, there are factors in real time that can make difference like possesion bonus...but even that cant be over 15%.
so my theory is that during that time before match when you cant do anything, dozens factors (str, moral, tactic, etc) from both teams are calculated + % that you can lose against weaker team (which is always possibility) and let's say after calculation.......it's 50% you will win, 30% its a draw and 20% you will lose.
You watch the game and one of your friends does, in front of your full stadium....and then you get 15% chance more to win...so now its not 50% to win, but its 57%.
so maybe changing tactics during the game does have some effect in changing the % of win...or maybe it's all calculated based on your starting tactics....but, hell...thats just my theory