Champion Rank S- S S+

What is the S rank criteria I played two very similar games today and I Got an S- on the first and no "S" on the second game(I didn't notice what my rank was). As far as I can tell I did better in the second match by far. My position was mid in both matches. The only real difference is that we took the nexus in the first match and in the second one the enemy FF. Getting more kills and more assists and less deaths and dealing more damage in a shorter amount of time isn't worthy of a S, this seems unbalanced. I know they don't want to just give these S's away but it would be nice if we could have a better idea of what will lead to a S. Also if the enemy FF before you can take there nexus this should not stunt your ability to gain a S. Match history first is the S- second is the not S? (http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2190884856/232106739?tab=stats, http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2191310918/232106739?tab=stats) Side note, anyone else think it would be nice to be able to see what your rank was for a match in your history so you can at least get an idea of how close you where because sometimes it doesn't load your rank and even if it does I don't always remember to look. Any Riot response would be much appreciated all discussion welcome.