16/1/8 Casa gets S-, 8/6/14 Kayle Gets S+

Isnt the ranking compared to other players that picked that champion?

Ehh, what about the other people in the game?

Not only does the grading scale scale you relative to every other (in this case Kayle) player, it also scales you relative to everyone else in the current game.

I had a dumb moment yesterday when I got this stupid ass score on Yi but only got an S- because our support got an S+ (granted, she was a great Janna that game so I'm not salty, but I'm just saying).

It's based on CS

While KDA is a factor in the mastery score, it isn't the only factor.

It's based on how people do with that champion. Clearly people generally just don't do well with kayle. Plus there's other factor besides your kda. Including C.S final build wards placed. Towers and dragon take downs and assist. ETC

Tip: Buy wards, upgrade your trinket, and use them. That will help your mastery score

I think it also depends on how your team does overall. Usually, you can get an S+ if you have a very high kill participation and consistent cs (meaning kills+assists).

Don't they count CS? (Jungle and minion being different) As well as wards placed, sweeped, purchased, vision and stealth. Possibly damage to both champs and maybe structures?