How on earth do these champion rankings work

I once played a game (and posted about it) where i was 15/2/9 and had an A+ and a rammus who was 9/4/15 had an S.

I was told that the reason is that you are compared to others performance on the champion, and thus an "S" was relative to what the average for a champ was.

Why don't people get this yet? Its been months since it was added. Its not just based on kills, you can go 20/0/30 and it won't care if you haven't placed a single ward, taken a single objective, got multikills, and if its on a champion commonly known to do good then its not going to give you even an S-.

Tbh the champion rankings is buggy

You can end up 24-0-30 and get an A+ or end up 10-6-2 and get an S

Riot should definitely fix it