Champion Mastery Rating System

I have tried for over a year now to try to figure out how the champion mastery system works. I have had games where it was clear why I missed S ranks (one less death one more kill or assist and it would be an S instead of A+). But tonight I have no idea how this wasn't an S.... 1/0/39 40/46 kill participation (87%) and that apparently is only an A. Sure I could have stolen kills from carries and gotten a better kill total but the job of a support is to enable their carries. I feel like I did the job pretty damn well as evidenced by the fact that my Graves got an S. I know I'm going to get flamed for complaining about this but I had to vent it out.