I got the feeling that Skill gains had a slight dependency on the form rating for that match, I see players that get a match rating of 9 receive much more than a player with a 6. I tried to keep check on this to see the player who had a 9 would decrease skill gain if he then got a match rating of 6.

Maybe somebody else can confirm? If that's the case it could be lower as playing a player out of position could negatively effect form if done too consistently.