Score in my game was different than Match History

I was playing an awesome adc game with Ezreal & had an awesome Blitzcrank support & at the end of the game I got a S- & Blitz got a S+. I go to my match history to watch the game & it shows my score as an A-. My stats were 15/1/5 & I did almost twice as much damage to champions than anyone in the game. Largest killing spree was 10 & largest multi kill was 2 but did that a couple of times.

There was a guy that was playing as Fizz that afterwards accused me of cheating & said me & my "friends" were going to get banned & that the guy I was playing with was a known hacker or something. Don't know if it had anything to do with that but I'm upset because that broke my S game winning streak...

Read some other stuff saying it might have something to do with the fact that the game was only 24:25 long or that they ff, but I don't see how that would change my score on only my match history.