If I create a new EMPTY layout 320 x 480 in the abstract designer and then switch between the empty and a non-empty layout, then the displayed background size is incorrect in the abstract designer.
Version 2.71
If you load the same layout twice it fixes itself so it depends on the settings of the previously loaded layout.
This is a visual good reminder (but not consistent) that they are not the same. I still think that is is a bug.