So thats a really quick potential flow model I made of the step noses. Imagine that the nose is outlined by the poorly painted black line that for some reason looks like a possessed dolphin.
Again, my MS paint skills are legendary! :P
But anyway this models the way that air moves around the body. See the very sudden dusplacement of the streamlines right at the step? And how they suddenly change thickness in several places and just generally look confused?
That is one of the mechanisms that creates drag, this change in the location and thickness of streamlines. And this model only shows one type of drag, it doesnt even account for things like surface friction or turbulence, which the step creates both.
however below you can see the smooth airflow that in effect goes under the car. This is a nice flow and as many have said this is what they were going for.
I unfortunately dont have the time to do anything better than this, but it gives an idea as to why the designers did what they did.