Confused by the grading system

Have been trying to get m7 on a riven recently and im getting confused by the requirements for an S. In the first game i went 8/3/4 with 7.6cs/minute and i placed 15 wards, 4 control wards and killed 5 wards. I lost the game too and i got an S-. It was a 35 minute game. Second game i went 7/1/4 with 7.9 cs per/minute and i placed 12 wards, 4 control wards and killed 4 wards. I won the game and i got an S- . It was a 25 minute game. Despite performing alot better in my second game in every aspect, time wise, i got the same grade and im confused as to how. If anyone could help that would be nice
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