Mastery discrepancy.

I was wondering if anyone has noticed a slight inconsistency in what is considered S grade performance in game, frequently i would see grades that were representative of my performance but in some cases i can't say i see the reason.

For instance whenever i play everyone's favorite scout i perform what can be considered above average and as such get graded well but i cant seem to find what is considered S- to S+. What exactly is expected of the player in order to earn these grades for some champs because it doesn't function the same for everyone. For instance the Kled in my last game earned an S going 9/5/10 which on average is a above average performance but when i go 7/0/6, 7/0/3, or 16/6/12 I consistently gifted with A+, what exactly am i supposed to do, are teemo players on average just that good for those to be considered below optimal or am i missing the point.