As @SuperDunk had mentioned above, There are 2 types of simulation engine in the game.
1) League, Cup, CL and Super League: main parameter is total quality of player(the value inside the parentheses.)
2) Association, Events and Friendly Champ: main factor is AvQ(%) of the player.
They are working different than each other. Im not really sure how the things go in #2, but if you have a player who has minus % skills in normal season, that means he probably won't be able to show his maximum potential. There could be a hidden multiplier like new association X0 factor. When this penalty gets involved over the minus skills, Player action in a position where the minus skills are important is completely neutralized and comes to a halt. At this moment, your player may lose the versus and gets an instant rating drop(red).
It's basically a nerf unit to prevent managers from dominating game by the mutant players. That's why mutant players rarely finish their matches with 9s or 10.