Um I got 16/3/14 
adc and the LoL grading system gave me an A and not an S whereas my 6/6/24 support got an S and I'm bitter. I carried?? (not that my 
support didn't deserve an S as well)
I don't understand how the grading system bases these grades, because on top of this score, I killed a tower, an inhib, and a dragon. (as in I got last hits as well as helping fight them) An explanation on their formula for grading would be great.
i'm too lazy to search for the post but a rioter said it takes into consideration:
-KDA -CS -warding -objectives
so you probably had low cs and low ward count
Its kind of like Lolskill.net's grading I think, its performance percentage based off the current average for that champion. And since Corki is face rolling everything, it gets harder and harder to get an S
KDA is only part of it. It looks at Warding, Wards destroyed, objectives taken (Towers, Dragon/Herald/Baron), ect. CS and all those factors. Plus the champs skill level also plays a great part into it. A lower skilled champ will get a better grade for having less then a higher skilled champ.
This is quoted from the Riot article about Champion mastery:
"It’s based on the champion and role that you just played in a game. So, for example, we look at Annie mid differently to Annie support. 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. The grade itself governs the point gains."
I think it goes of of things like how much damage you did and not just your kills, it also has to do with how much damage you tanked and your assists. If you die a lot you won't get an s and if you have 22 kills and 4 deaths you aren't guaranteed an s as that has happened to me too.
