A question on determining match performance grade (with Match Histories)

So, due to the update on the Mastery Levels, i've been trying today to get -S rank or higher so i can get my main champion to Mastery Level 6. However, after five plus games and only one S rank, with (in my opinion) increased performance each game, the only S rank is still the first one (also, rank decreased each game, more reason for my confusion). I've looked at at least four of these matches (one was... irrelevant... i went AS Soraka mid because someone else wanted to support) and compared KDA, damage dealt, total gold, CS AND warding for each game. Here are the links for the first match post patch, and my two most recent matches as of posting this:

First match (rank -S): http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2190411858/226196766?tab=overview

Second-to-last match (rank A): http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2191127897/226196766?tab=overview

Last match (rank B+): http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2191174866/226196766?tab=overview

Here is a link to the OP.GG page for my account, which is what i used to compare the stats for the various matches (as i could compare them all on the same page, and just scroll): http://na.op.gg/summoner/userName=n3razim+pra3tor

the first match is the one at May 18 2016 10:02 am, the most recent (in case i i play more immediately after posting this) being May 19 2016 12:21 am. The first two games that i used for comparison were blind pick, while the second two were draft pick in a four man pre-made, all were normals (in case this matters). If anyone can help me understand the difference in rank between these games, as well as any tips for improvement (outside of item build, lol... no offense, but i'm rather comfortable with my Soraka builds), i'd appreciate it.

TL;DR: How does match performance grading work? i played games where i believe i did better but got lower grades.