What qualifies an s+?

Okay so I have gotten s and s+ games before but in the past few days I have been doing quite well as Kassadin{{champion:38}} and getting only B and B+. For example my last two games I went 17/6/9 with 118 cs and 17/2/9 with 76 cs and I only got B+ on both. Could someone clarify as to why these games are not s or at the very least A+. I get the cs is kinda low but still I carried in both games expecting definitely above a B. I'm pretty new to the game and its just confusing me.
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