Re-reading my response, it was a bit fiddly. However it has the full explanation of why you'd want to do this: * Add a UVW Xform modifier with a U scale of 0.707 before adding the Unwrap UVW modifier :)
UV space is purely described as a 0-1 numeric range. So even if you specify a different width/aspect ratio for the display of the UV space, the positions/values of all the UVs are still the same (think of it like a percentage - a UV point at 0.25 will still be 25% across the space, regardless of if it's square or…