Champion letter ranking inconsistency.
Recently played a game as twitch and I went 15/0. I didn't get an S... In fact I got a A- I've gone 4/2 with certain champs and gotten an S, so why by going 15/0 I don't? It didn't really matter because I already earned all my hextech chests but if I hadn't this would be really annoying.
Rito pls I need that S consistency

EDIT: Match history :http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2131104615/212457877?tab=overview
Also, this may be in the wrong board section as it isn't much of a bug.