Getting an S is more than just a matter of snowballing.
I get an S- or greater almost every game I play as Vlad.
You have to be above average in all aspects.
Gold, CS, K/A, deaths, damage taken/damage given, healing, etc.
Notice how most people that think they should have gotten an S in whatever game they have a decent KDA in have pretty low CS, or something else that's astonishingly low.
Getting an S is not confusing. Using your lead properly will get you an S. Trying to snowball on kills will not.