Backspace and offset dont seem to make sense, US rambler mags

Hi,
I have been looking at the US rambler mags wheels for my car but I cant figure out if they will fit or not. Link to the wheels:

Rambler - U111 - US MAGS

It says that they are ex. 8in wide with 4.5in backspace and an offset of 1mm. How does this work? Should the offset not be +1/2in or +12.7mm?

From my understanding offset is a measure from the wheels centre line to the mounting surface and backspace is a measure from the inside lip of the wheel to the mounting surface.

If an 8in wheel has a backspace of 4.5in this puts the mounting surface 1/2in from the centre line of the wheel, meaning the offset has to be +1/2in or 12.7mm not 1mm. Where am I going wrong here?