How do lol grades work?

So until 2 days ago I was in a constant A. Until today I was on a horrible loss streak and I was getting C and D constant. A score of 2/7 does seem like a D score, and 7/1 with 88cs doesnt seem like a B score, A- minimum. I have also had games where i went 2/13 and I get A but a 1/6/10 score with 100cs I get a C. Was this recently changed or is there something im unaware of? Please let me know and thx.
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