I recently played a game as 
in mid facing 
, mainly to counterpick and try to get a 2nd Mastery 7 token on . However, even though I went 13/0/12, I still did not get a Mastery 7 token. I only got an A-, which is stupid since I never died. The only thing that I can think of that would have kept me from getting it is that I only had 141 minion kills. I just think that it is stupid.
CS is probably one of the highest weighted stats when it comes to performance assessment from the system.
If I were you I would work intensely on your last hitting and I guarantee you you'll see some more S ranks in your future.
How long was the game?
Mid should have the highest CS in the game, and when you had a 25 kda, with only 141 minions, sounds like you didnt farm very well, and just kept killing a kat who wasnt worth any gold over and over.
Your farm/CS was probably too low. It counts just as much as the kills and assists you get throughout the game (If not more).
