Uhm..?

I played a game and earlier today and went 6-0-42. I understand my CS isn't high at all, but it's not supposed to be as supports. I was wondering what the requirements are to get S because I definitely thought I'd get an S on that game, but only got an A+ Is there an official formula to how the S is calculated?

What confuses me even more, is two games before, I went 4-1-25, and got S+

A+ http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2556220801/239359879?tab=overview

S+ http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2556126630/239359879?tab=overview