[USAnum1]

That doesn't have to be true. The NY examples can hold true, and the center of mass for population can move west, while overall population density still climbs. For example, the east coast cities could change in no way, while midwesterners moved to more dense western and southern cities (even if they are less dense than the East coast, it's still an improvment). This would result in the population shift you identified, while still restulting in an increased overall population density.

WRT NY's density falling, it could be an outlier or the norm. Still, from what I understand overall population densities are increaing in the US. [1]

[1] http://www.nytimes.com/2014/04/17/nyregion/suburbs-try-to-ho...

edit: Clarification and spelling.