What makes 1 game an S and the other not.

First game got an S http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2214326346/35540404?tab=stats Second game got an A+ http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2214402277/35540404?tab=stats

I was support lux, both games are pretty similar but I did better in the second one and got a lower rating?