38/2/4 K/DA, only A+?

So, today I played a Draft where during champ select, some unintended beef happened between me and two teammates and they decided to feed right away. They fed some 53 kills to the enemy but I managed to carry my team and pull off a 38/2. I thought Iw as finally gonna get mastery level 7. But then I didn't get even an S-. It was an A+. How the hell is this? I have performed much worse before and got many S- ranks, how come I get this now? Did I do something wrong?
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