If you look at my match history, I'm getting an S nearly half the time for the games I won. I've never had trouble getting mastery 7 in a relatively short amount of time and seem genuinely confused when other people do.
http://matchhistory.na.leagueoflegends.com/en/#match-history/NA1/236297233
Leylania1 votesI think it's mainly because of the CS. You seem to farm pretty nice most of the games while other people like me (yeah supp main here sorry) don't farm that well. When I play mid I often don't get S for exactly this reason, but when I have nice farm i get almost always an S aswell.
marpatt111 votesa lot of people don't know this (that I know of) but I feel supports are easier to get S's on, all you need to do is have low deaths and at least (iirc) like a few assists, another way to guarantee it is to get like 1 or 2 kills with like 1 death, that will most likely get you an S, this is one of the reasons I main support, no need to farm, you help the team, and 
Hallbuster1 votesI think it's mainly because of the CS. You seem to farm pretty nice most of the games while other people like me (yeah supp main here sorry) don't farm that well. When I play mid I often don't get S for exactly this reason, but when I have nice farm i get almost always an S aswell.
Thank you very much, I take pride in my CS'ing skills!