Wish There Was More Transparency With Mastery Grades

So this isn't some whine thread, but more just trying to understand it, and more so wishing it was more clear.

I understand the base of "your score is based off of the average of x champ based on other players results". I've also heard it is based off of the scores of those in your division as well, but I haven't seen that confirmed. I'm just starting to find it frustrating, but usually just ignore it (Because it isn't really important), when I can't figure out why I got S on a meh score, but A on a score I wish I got more often, it makes me scratch my head.

I've gotten an S with less CS, and like 4/1/16. Decent score, sure, but then I play a different champion and go 17/5/17 with maybe 20 more cs, and I get an A-?

I very seriously doubt there are a lot of Rumbles in silver, let alone with decent scores. so I just wish I could see "behind the curtain" a bit and see what actually calculated to give me said score.

I mean, maybe it was my CS, but we were going full on clown fiesta killing the enemy and taking turrets/objectives. I dropped a few pink wards and warded tons (yellow then blue). I warded LESS the game I got an S with lower cs, kills, and assists.

tl;dr just wish I knew what I was doing that impacted the score, because I can play the same champ 2 games in a row, get a similar score, cs, wards, etc, but somehow get anywhere from a B to an S with minimal difference. (only reason I even care to be honest is because I want rank 6 and 7 with champs I have like 108k and and 72k mastery points with (play a lot of support).