i think it is just exporting all four layers of the background, some parts appear behind you, others appear in front, and it uses the layering to do that (if it were one image, it would have to specifically tell the game which coordinates of the picture would be in a different layer, and that would take a hell of a lot of programming, so it is in layers instead).
when editing the backgrounds (for when the import ability arrives), it would need to be done to all four layers, only smoothing/changing one would not do enough.