Got an s- but I dont know how

So I got my first s- as zed but I dont know how. I went 6/5/9, had 145 cs, Placed 4th for dmg done, last for dmg taken and only got a few towers. I just wanna know how I couldve got an s- when I clearly have had better games in the past but got below an A. The link to the match is given.