No it's not a hard concept to grasp at all and amazingly you failed to do so.
You make the false assumption that because a player starts from the bench in a particular game he is therefore incapable of playing 80 minutes.
The correct way to determine if a player can play 80 minutes can only be achieved if the player actually starts the match.
Even then allowances need to be made for
- Was it the coaches intentions for that player to play 80 minutes that particular game or did he formulate a specific game plan for that game?
- Was the player subbed out due to injury and if not for that he would have played 80 minutes?
- Was the player subbed out early as a reward for his efforts and the fact that his team cannot be beaten in that game.
None of those scenarios is an indicator that the player cannot play 80 minutes and as my previous post clearly shows AM completed 7 out of 8 games (87.5%) where it was possible for him to play 80 minutes and only a season ending injury stopped it from being 8 out of 8 which would have been 100%.
As George Dragon correctly said "averaging" was a bit lazy and you have taken that to a new level.