Grading system - Question

I just played game Karma supp 2/3/23 (36 kills total for my team) and i got A+. My jungle Warwick got S (9/2/13), same as Tristan (5/7/15) and Jayce (12/6/6). When introduced with new system, one of things they said was this (in post about grades and masteries): "We take a series of performance metrics and compare how you do to all other players in your region in your champion / position combination. It is percentile driven - so earning a high score means you performed in the top x% of players on that champ in that position." So can someone explain to me what exaclly one has to do as support player to get grade S? I dont believe that I am top player, but still I dont think that Tristana's score is one of top x% of players on that champ, and still got S.