Well yeah, the fundamental difference between WiiU and PS3 though is that retailers want to stock a lot of PS3s because people buy it. With WiiU there is no guarantee
With it selling ~50k/week I'd say 500k on shelves does sound about right
But the retailers aren't keeping low supply of the system. They get more before running low. They just don't flip them as fast and stock sits around. If each target has 10 in stock (that's low) then that's 20k in Na alone in one chain. So those ten will sit around and then they sell through 6 in a month. They will get those 6 back before the other 4 sell. Again, there is a min amount retailers will be able to have before being supply constrained. If each store has 2 units you'd see a lot more sell outs. (hell even 4 or 6).