S on Xayah

I played a game of League, me and a friend played Xayah and Rakan. It was fun and i was dominating. I ended with 14/0/9, with 150 CS,and ended at 31:05. My vision score was 19 that game with 13 ward placed. I got an A-. My and my friends I played with don't understand why I didn't get an S or even an S-. I don't understand this, I legitimately feel cheated out of an S.