How?

I guess other talons are just better?

KDA isn't everything.

I looked up the game you are talking about and you just had 118 CS in a 27 minutes game. I can't stress this enough: CS is an insanely important factor for your grades.

You can have an absolut average KDA, as long as you have top notch CS you will get that S grade. But if you have underwhelming CS, then you won't get near it.... Try to work on your farm, get as much CS as possible and you will see an improvement in your grades over time.