What metrics are supports graded on?

Ok so some background. I play mainly 4 champs: Lissandra, Nunu, Orianna, and sometimes Kennen. I play them all as supp in normal games (I don't play ranked). As far as I've read, mastery grades are determined by comparing how well you do to how well everyone else who has played that champ in that role has done. Assuming that's true I was wondering by what statistics supps are graded on.

I tend to have a better kdr and significantly more assists as Orianna than the other three champs yet comparatively rarely get better than S-. The only significant difference between Orianna and the other three is that I play Ori for her shield so she ends up doing significantly less damage than Liss, Nunu, or Ken. Is damage shielded not a tracked metric?

If I am to understand it correctly this basically means that either I'm bad at Orianna or (more likely) the other people that play her are better than the others that play the other champs mentioned. What I'm mainly wondering about is if my play style (which might vary significantly from other Ori supps) and in turn my performance may not be correctly reflected by the grading system since I focus mainly on utility/damage shielding instead of chunking people's health which is what Ori is often used for.

A couple additional questions. Are grade comparisons separated between ranks or is each player compared to every other player (of the same champ in the same position) regardless of rank? Lastly, is distance to a skirmish/time spent near fights a tracked metric? Many of my most clutch plays tend to happen when I'm relatively far away from the action or just get somewhere in time which means I don't spend a lot of time physically in the fights.