This is a game my friend just played... note the Vayne who got an S+ with a score of 1/0/0, and doing only 757 damage to champions. Has 128 cs in ~18 mins, took 3 turrets, 0 dragons/barons. How was this an S+?
Edit: Concerned about an exploit since this guy said he got mastery 7 Vayne from this. And he has 17 mastery 6+..... as a Gold 5 (0 LP) player.
Troll statistics. Ive had games where Im the best performing champion in the match and get A-. You name it! most kills, most cs, least deaths, most support kills, and leading first or second in damage..... awarded A- 
.
At this point im thinking its kinda random. The better you play, it cuts off lower grades and awards you something random higher up.....Also, when your kills get ridiculous you get awarded free s- or greater regardless of any other stat. Dont count on support kills ive gone around 6/3/40+ and earned a filthy A. What vayne got there must be from the lack of deaths and damage output.
I didn't think damage was actually counted. I was under the impression that it was:
Objectives taken
Kill to Death ratio
Wards placed
CS, scaled to meet the game time.
vayne had highest cs, and probably did most damage output and took decent damage. He also probably wandered a lot more, had more wards down, and applied constant counter pressure and taking objectives during the team skermishes.
I got an s+ (as only member on my team with S rating) with rek'sai on a score of 4/4/10 (with lowest kill participation) for those reasons above.
Note: I almost always get S rating on games I lose.
