This has to be a bug.... I can't get an S rank, at all...

Here is one game example to go with the description below. I have calculated time to cs with 4.3cs a minute. Game time was 33 minutes 30 seconds.

So I have been playing Kennen as my main mid/top laner on the NA public servers in patch 6.11, and have kennen mastered to lvl to level 5. All my friends on my ranked team have their champions they play to lvl 6 or 7, and no matter what I do, I cannot get an S or S- on Kennen. I tried in a pvp, and played to a 9/3/2 kda, 162 cs, towers, a dragon, and inhibs. I got an A+?! Can someone please help me fix this? I dont know if its a bug or is it impossible to get mastery 7 Kennen? Please help riot staff! D': ~Gardaen Patch 6.11 Bronze 3 Mid/Top Main Kennen

EDIT: In case of unclear wording and confusing comments, the game was a blind pvp where I got: Kda: 9/3/2 Cs: 162 Time: 33 minutes 43 seconds Cs per minute: 4.3 Pinks wards were bought. At least 25 that game. Least deaths and 2nd best cs that game.