How Can I Get An S Rating in ARAM?

Just played an ARAM match. Very successful. Went 13/1/15 with . Had 45 farm, did 18,830 damage, 9 kill spree, and highest multikill of 2. I received an A+.

Now, my team had a good too. She went 9/2/13, had 22 farm, only dealt 9,862 damage, 4 kill spree, and highest multikill of 2. She received an S-.

I don't think I am out of line when I say that this makes zero sense. Is it because she was able to purchase more items than I? If so, I apologize that I played well and didn't die more than that once, but that cant seriously be the case. I know that there is often a lot of questions regarding how to receive an S rating and was wondering if things are different for an ARAM match.

If anybody is able to lend some knowledge or if anybody within Riot can help address this, I would be thrilled. This just irks me in all of the wrong ways. Thanks!