Originally Posted by N1234

Think of ORD as the holding pad for new pilots / reserves. It is generally easier to get people to move out west than the other way around. So there is an artificially high reserve count in ORD as compared to other bases.

I haven't checked out the details but there may be a case where an ORD reserve FO could hold a composite line out west but for whatever reason isn't willing to transfer (most likely he or she is local to ORD).