Working at a Peds shop with pediatricians, EM, and peds EM (all peds), I can venture a guess as to why a corporate group would pay EM docs more than the others.
It's because they work faster and bill better. That's pretty much all there is to it. It is annoying to be called the adult doctor by the staff, but the nurses still know who moves the meat and who to get for the sick ones.