mastery questions

So just got out of a game, i went 12/0/7 second highest cs in game, i had a 135, our mid had 138. I won in every stat otherwise. Best KDH, largest killing streak (12), most multikill (3), highest damage to champs and over all. (21,464 and 95,185 20 minute game), most wards placed (tied with support for 13), most gold (10,099), most towers killed (3), and most inhibs tied (1). I got an A ranking.

Now our mid lane, 138 cs, 7/1/6, 7 killing spree, 2 multi kill, 11,102 damage to champs, 75,239 damage total, 10 wards placed, 9,384 gold, 2 towers killed and tied inhibs (1). he got an S ranking.

How does it measure this mastery thing. I literally was higher in every stat, and yet got a lower ranking. It feels completely random.

Just wondering if anyone knows how it works, or why this would happen this way.