Why can only one inner region of a plane be subdivided? Starting from this situation:



Subdividing W the inner rectangle leads to the following model, please notice that the center points of the adjacent rectangles have been displaced. Subdividing other rectangles is no longer possible. 
To analyze the behaviour, I tried to subdivide the outer regions first this works flawless.

Why is that?



View from vertex mode:



.blend
The outer faces are now ngons. When you try and subdivide the outer faces they are being subdivided, which you can see when going into vertex mode:



The subdivision can't happen like before because while the faces appear to be rectangles, the topology is no longer quad based.