@Riot, Mastery rating system

Hello Riot, I have two games for you to look at and explain the ratings.

Game 1: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2219798560/35903889?tab=overview

I made many mistakes, died pointlessly many times, was largely carried by my teammates and I got an A rating.

Game 2: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2217992571/35903889?tab=overview

I was nearly flawless (for my low elo anyway). I had no deaths, had the most damage to champs in the game even though I built tank. I had the second most damage taken in the game, despite no deaths. I was at nearly all the tower kills, both dragons, and Rift Herald. For this game I got an A+.

So one game was pretty poor and the other was exceptional, at least for me. However, according to your rating system it was basically the same performance. How does that make any kind of sense? I am starting to feel like your ratings are completely arbitrary.

Edit: Corrected second link