Apportioned plates mean you pay each state for the portion you run. Some states are more than others. Sometimes it's cheaper to add states from low cost states to offset the high ones. In my case, by doing that the price dropped from $1400 to $ 900.
If you don't go to the other states on your cab card you have to pay a penalty when you renew unless you remove them. Basically, each year you have to juggle states around if you want to outsmart them.
If they change to the new plan Gman is talking about it will probably change the juggling strategy.
According to OOIDA, the reason so many carriers are in IL is there are political deals behind the scenes.