Like demand exceeding actual manufactured products ?
Anyway it seems that the shortage will last for a while http://www.joystiq.com/2010/02/12/sony-ps3-supply-to-be-tight-over-coming-months/ , but hey Sony could always use some of those 3 million PS3 sitting on the shelves to suplly NA stores.
could be a lot of reasons. It could be that some retailers have a huge amount of stock. It could be a revision with new hardware. It could be a price drop coming up. It could be a stockpiling to deal with the next 2 months of huge game releases. It could be that sony is counting units sold to the airforce or other institution. It really can be anything.