Why didnt i get an S or even S-

Okay, I am outraged because we just destroyed in our silver rank game and I completely wrecked my top lane opponent (eve) yes i know eve top shouldn't be hard to beat but i stomped her. I was 13/1 had first blood, had first tower blood, most wards placed, assisted with baron, got my lanes inhib and the only thing I didn't do great was my CS because as soon as eve came to lane I would attack her and kill her and then sometimes had to base because I'd be low health and didn't want to get ganked. So you're telling me that because I was stomping my opponent so bad that I lost out on some CS that I don't get an S when 3 of the other people in the game did. This was my fourth or fifth game with gnar and I am just so mad at this crap.