3-6-18 blitz support and no S?

It's very simple. Your KDA is NOT the only thing that matters.

Grades are determined by

-KDA -First blood (this one helps A LOT) -CS /Total Gold overall -Warding (Placement and Destroyed) -Objectives (Towers, Inhibs, Dragon, Baron, Herald) -Skill level of the champion (does the client show it as high or low, the higher the more it takes)

the system is apparently hidden to prevent abuse. You more than likely needed more CS, less deaths and (my own personal belief) a last hit on a tower.

Its random. No seriously it is. You could go 900/0/900 and get a C

that's ~1 death every 3 minutes in a 20 minute game

i counter with a Jhin 3/0/5, 72cs/10 got an A- and yes, we won and yes, i warded

The system is also dependent on how well other players do with the champion. Your score may be good, but if it similar to other scores people get with the champion you will likely get an A.

Who knows though, sometimes you get weird results. Over the weekend I played Veiger and went 8/4/6 in a 48 minute game that I lost and still got an S-. Granted I had 367 CS (spent a lot of time farming/defending in base) but I was still surprised when the chest popped up. I didn't even think it was possible to get an S score if you lost the game.