I Got an S+ How and Why? O,o

I had so many mechanical misplays and barely even completed one item. Aatrox, Soraka and Master Yi affected the map a lot more and the enemy team had a disconnected person from the start. I've had good games, but that did not feel like a fair S+ or a very good game for me (mechanically at least). I won't complain about getting an S since they're so rare for me, but dang. I'm confused.

I only barely did the most damage on my team and yet nobody else on my team got more than an S-. Damage: http://prntscr.com/h471sw