17/0/6 A+ ?????

everything counts, KDA is a really small part of the grading system, how much CS did you have? wards? objectives? ROAMING COUNTS TOO

You aren't graded just for getting a bunch of kills, you're graded for being an overall good player. That means CS, Wards, objectives all count.

Your CS was garbage.

Along with all the other things already mentioned, it also takes into account how long the game lasted, and how you compare to other people who have played that champion. For example, if you have 100 cs in a 30 minute game, you are pretty much guaranteed not to get an S, unless you are playing support