How does the grading system even work?

Im confused on how the game decides what rank you get (A, A+, S, ETC) I have had m6 teemo for the longest time and recently I have gotten amazing KDA on the champion. Im just confused on how this game https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2693504221/237968169?tab=overview and this game https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2692826677/237968169?tab=overview both give me an S-. On the second match I listed I wasnt really that confused about an s- but when I got s- on the first game I posted I was kind of confused on why it gave me such a good grade for that score. I would just like a deeper understanding on how you are graded in league. Thanks to anybody who can explain to me the logic. Also Im not sure but im almost positive I got an s- in this game as well.