The S-grades algorithm for ARAM is confusing as hell.
I once played a game as Ezreal where I had really good KDA (luck in champ select), barely 5 or so deaths, certainly over 15 kills, many assists since I was barely dead and still got just A+.
Just now I played as Ryze with 14/10/29 which isn't impressive at all except for the assists which also aren't impressive since it was long game and it was S-.
I realize it compares with results from other players (which is already dumb because you might play against bunch of hard-counters and get compared to someone who might've went 20/0 because they had perfect comp), but I think both Ez and Ryze are quite easy to play in ARAM, yet I somehow had to try less with Ryze. Much much much much much less.
Now here's the thing. 102 minions and 18k gold doesn't sound impressive on its own, but I certainly cleared minions more as Ryze than Ez. I suspect Riot legit just copy-pasted the algorithm from Summoner Rift and you get S grades easier by just having strong waveclear in very long game.
The algorithm just confuses the shit out of me.