Can a Rioter explain the Champion Mastery rating system?

This isn't the first time that I think I am going to get an S on Blitzcrank but I don't because of reasons? I had 72% Kill participation, 1 death, plenty of wards placed hell I bought 5 pinks that game as well. My only thinking is that I didn't do enough damage because I went full utility which feels shitty if that's the case. I mean it feels terrible that I don't know how I am being graded nor does it help me to improve in certain areas.

Edit: I got an A this game.