Lets say Player 1 and Player 2 have the same skills, GS, etc, but have different stamina levels:
Both players get 60 minutes in the week:
Player 1 plays 48 minutes in one game with low stamina which makes him have a bad performance.
Player 2 plays 30 minutes in 2 games with strong stamina with a good performance.
I have noticed that player 1 will not have a great increase in GS even though he has played 60 mins. Player 2's GS/DMI would be higher. I think somehow the player's performance is slightly linked to GS. I could be completely wrong, but its just my theory.