What affects your GRADE? S+ or D-?

@Riot Hey everyone, I just went 13/0/10 (128 cs) in a 25 minute game as Irelia I participated in most of the tower destructions, and as you can see my score isn't bad at all.

So I was wondering, what really does affect your grade? Why did I get an A+? Why not an S? What was the factor that resulted in my grade being lower than expected?

People say it's because it compares you with others from across your region, but how come some people are still able to go 20/0 and not get a good rank? It's not like you see a super fed champion every game that never died throughout the whole game.

Thanks

• Your1Opportunity