Tilted

I got 21/0/4 in a game and my friend got 12/5/4 but we still got the same letter grade, S-. If you want the other stats of the game go to http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2549168227/207602648?tab=overview. I feel that the time of the game effects your grade because I can remember a game when I went 6/2 with twitch and the game lasted 40ish minutes we lost the game but somehow I got an S-.