It's probably about Ratios.
I don't know the formula or algorithm that Riot uses with their grading system, but I am almost positive that it involves ratios for BOTH KDA and CS.
And this Ratio is probably related to the TIME in-game.
Ex: to get S tier, player must have X ratio of Kills and Assists to Deaths at time A, but also must have Y ratio of personal CS to total CS spawned by time A.
Your KDA was obviously better than Shyvana's, but your CS probably didn't hit the benchmark for S tier. You probably needed 20 more CS to be S- and 40 more CS to be S. And probably 80-100 more CS to be S+
Your Kog'Maw probably got A or A+ but obviously didn't get as many kills as you.