It's just the way the thing works, apparently.
Each bitplane stores a run of 8-bit pixels. If you use a single bitplane you get a 160 width screen, with linear access to the pixel data. If you want a 320 screen you use 2 bitplanes, alternate bytes on each plane. For a 640 screen, you end up with four bitplanes, alternating pixels on each plane.
All of these are using SHRES displays on AGA. I can actually see that a 160 wide screen composed of 8 bit pixels would appear as a 1280 wide bitplane since there are 8 bits to each pixel. No doubt in order to be displayable you need to use the other bitplanes to bump up the resolution since 1280 is the widest displayable width you can have.
At least that's how I understood it.
