how riot rates you game

ok, so ive had games going 18/0/4 with 120 cs in 18 minutes and get an A, or 6/0/7 in 250 with 170 and get a B+ like even if i play a perfect game i cant get an S, and its honestly pathetic, i love this game but its pissing me off that i play the perfect game and cant get a mastery token, plz riot if you really care about your players you need to work on this, like i dont even wanna play anymore bc i know nomatter what i cant get an S.