Hey there! I've been playing the game and also visiting the forum for awhile now and I'd like to share a few ideas that I got about the possible reasons for player (under)performance.

When I first started playing T11 I thought that the important factors in the game are 1) Player stats and 2) Tactics and Counter formations.
But as the seasons went on (I'm now on level 13) I realized that some players with higher stats are performing worse than other, supposedly weaker players, not only in a single game but for 5-6 consecutive matches (morale and condition were maxed). So I thought about possible reasons for that and got the following three ideas:

1) Probably a player's performance is not a question of the level of stats as much as their balance (between attacking, defending and physical). That would explain why most people consider Nordgen players to be better than others although I don't really agree with that (I've had good and bad Nordgens same as good and bad transfers and youth players).

2) Another idea I got is that maybe some players are suited to play well in certain formations and not so well in others (I mean in general, not as in counter formations). For example my best striker seems to perform amazingly well in setups with only one striker and not so well in 2 ST or 3 ST formations. Same could apply to midfield and defence as well...

3) The last possible reason that I thought about is that players could have some hidden specifications (using the same logic as special skills). For example player A could have a higher chance to get injured while player B would only perform well until he reaches a certain age, like 24 years or so.

Last, I want to note that I'm not dissatisfied with the game at all, I just want to see what other people think on that topic.
IMO the fact that the game is sometimes unpredictable and not just about superior numbers is the reason why it's interesting and kinda exciting.