While I agree with what you are saying regarding CS - I am within 50 total CS of every other person on the map .... and its hard to get CS when you are constantly roaming to top and bottom to help secure kills - as you see I also had 18 assists.
What I dont understand is how Sona got the S- .... I have 10 more kills ... 7 less deaths .... 8 less assists .... 156 more CS ( i get she is support and shouldnt have cs ) - and roughly 2k more gold than anyone else on my team.
Just doesn't quite make sense to me. had I gotten these same stats as a jg - I would have gotten an S+