How do they rate a game?

So I just finished a game where I went 27/8/9 with khazix. I feel like I played very well I had the highest damage in the game against an 18/6 jinx (pretty sure I was her only deaths). I don't understand how the game decided to give me an A + when I have played games where I have gone 11/8 and gotten an S for them on the same champion. I'm not whining I'm just trying to understand the rating system so that I can play accordingly. Do you have to win to get the S? Is farm that important when taking a player off of the map lets you take an objective? Is being there for a tier 1 or tier 2 tower that important? Once again just wondering because my farm was not atrocious. Honestly I would just like answers I felt like I played the best Khazix of my life and to get denied like that led me here.

-TYVM Evilbudditch