Grade system

Could be anything. A little bit more CS, warding, kill participation, objectives. Whatever metrics they use, they are each given a score and averaged most likely (with S+ being the max for each) so any category under S+ would drag the total score down some.

Kill participation was the issue here. Possibly objective contribution, too. That said...

I think I've only gotten 2 S+ grades. Normally, the highest I get is an S-. It's pretty hard to communicate with your team to actually pull off an S+ unless you're using VOIP.

The system gives you a grade by comparing your post game stats with those of other players / games. It just means there are Yas games out there were people performed even better. That's it. It in no way looks to compare how you did on an individual game. Just comparing post game stats of your champ with the stats of other games.

Be slightly better. XD

Theres really no reason to get worked up about this, according to the score you got you did pretty damn well, its only a matter of time before you get there.

But since you are so good with that champion and Grades are relative to the role you play, perhaps consider playing him in a different role. If your champion isn't usually played in that role then chances are you'll get a higher rank. It's worth a shot.