It's not just "more than KDA", champion grading simply sucks.
I'm sure it's decently accurate on solo lanes but as support or in game modes like ARAM or hexakill, the grading is way off because of the CS factor in the calculations. For example my latest game.
An enemy went afk on lvl 2 so their team asked us to push mid and so we did. Result? Everyone in my team gets S- or S, except for the support, who gets B. Why? Because the support didn't get CS.
I even went B to get sightstone to place more wards, a pink ward and red trinket. I shielded nearly every hit that my teammates received from poke or turret shots and I had the 3rd most damage dealt to champions.
Now my point isn't that I should've gotten S. Imo no one should get S in a game with an early afker. But why such a discrepancy between scores without any explanation besides the lack of CS on a support?