S Rank Shenanigans

I just came here to vent because I've been playing Shyvana non stop trying to grind that last S rank for Lvl 7 mastery. I played a game recently where I went 4/0/5 in 20:30 with 132 creep score and got an S-. As frustrating as that was I was determined to continue playing. Today I went 12/1/10 in 25:11 with 135 creep score... and got an A+. UUUUGHHHH. 3 times the kills, 2 times the assist, 2 extra creep score with ONE extra death and I get scored lower. Is this because I wasn't farming enough? I know that is one of my personal flaws, not farming enough. But only 5 more min than my S- game.... seems a little lame I didn't at least get S- again...