Quote - I don't believe these are quite the same as normal maps, since normal mapping requires three colour channels and .BUMs have only two. The blue channel is unused; or rather, it produces unusual results if you hack one to add stuff to it.

I suppose they are local normal maps, ie instead of storing the full 3D normal they just store the delta normal in local space. In which case you wouldn't need the 'z' component cause its not as if you can move a normal perpendicular to the surface.