Why no s

Your rating is based on your performance compared to others playing the same character, and it's based on CS as well as K/D/A. It's not that infrequent for a Yasuo to get fed early and put up big stats, so 15 kills with 7 assists may not be an S-tier scorecard for a Yasuo. Also, I don't think the performance rating is checked against game length.

You think that's bs? I got 4 s- in a row! Guess what? s- doesn't count as an s! So I still can't move to next mastery.