Hi !
I was just playing a league game (Diana
) and my score was 24/7/13, i had 20K. (Most on the team).
ofc. had full build and all that stuff.
-
i was just wondering if somebody knows how the system works 
?From your summoner-friend 
How long was the game? How much cs did you have?
Usually a decent 4.5-6 cs per minute for jungles with a good KDA results in a S rank.
The mastery system compares your performance to other people playing the same champion, probably in your same tier. To get an S you have to significantly outperform the majority of games with that champion which is determined by a large number of factors. Kill count alone doesn't matter that much.
if you only had 181 CS in a nearly 50 min game, that's why. around 50 min you want around 500 CS
