Why the mastery system is not rewarding S' properly.

So, I just played a Normal Draft game as Lucian. We were losing hard, top lane and jungle have 6+ deaths and 1 or 0 kills. Then I just started farming getting picks around the map and I suddenly have 10 kills no deaths. I obviously didn't want to die, so I stayed back. I didn't wanted to die because if I got shutdown they would get the gold and my chances of getting an S is gone. Then by the end of the game, my score at the end was 17/0/9, with 337 CS. I was hoping for an S because my teammates said I would with the farm and score I had. Then I got an S-. I don't know how to describe the feeling I had when i saw the minus on there. The game before that, i go 4/0/2, 98 CS and I got an S. That game ended before the 20 minute mark so they couldn't FF. So, I'm wondering is how hard do I have to carry to get this S? 30 kills, 500 CS? Cause it sure does look like it. If you don't believe me check my profile out which is, "cehjay." I play on NA servers. I'm a Bronze 2 player but I don't care about my rank. I just want my S.