The selectors arguments dx dy dz take the delta x y z of a cube to search in, not coordinates.
The description, however says "Entities between x/y/z and dx/dy/dz" which makes it seem like they are separate coordinates.
It should instead say "Entities within the cube of size dx, dy, dz from x y z" or similar, indicating that it's actually the slection box size, and not separate coordinates.