What's with this grading system?

I went 0/6/37 on Janna with 66% kill participation, dropped more wards than anybody in the game with my total being 52, earned more gold than the enemy support, jungler, and top laner, healed for the most damage in the game at exactly 30,133 damage healed, bought a Ruby Sightsone and Redemption, and only got an S-.

dafuq?

[zombie-brand-mindblown]