How does Rito grade us?

I am genuinely wondering what the algorythm is to calculate our grade at the endscreen. This is because I had way too many games for which I am certain that i should've gotten at least an S- but it always ends up being an A/A+ or even A-. Let me give you my most recent example: I had a game with Kha'zix where i single handedly snowballed all 3 lanes and made a Syndra ragequit (he was a nice guy I said sorry after), here are a few stats. KDA: 12,0 (8/0/4), 52% kill participation, 5/minute cs, 9 wards and 1 destroyed in 21 minutes. Why is this performance not an S?
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