Match Performance Grievance

Okay, I main Vel'Koz. I've run into a bit of a problem... After I get around 100 or so cs, I turn into the primary champion damage dealer for my team more often then not, and do quite a bit of roaming rather than farming. In this way, I support the team more than farming alone.... My problem is, although I just finished a game where I was 15/3, and have had MANY game where my k/d ratio was higher and had more kills, I can't get even an S- because I focus on team contribution over cs. I can't be the only one having this problem, and I feel that this needs to be addressed.