So how do rank scores work?

I have no idea what is happening in the rank score after the match. How is this even remotely calculated? I had back to back games: game 1 I go 10/0/5 with brand and get a B, game 2 I go 5/2/2 with Fiddle and get an A+. I get it isn't only K/D/A but I mean what the is this. Any insight to this process would be great.