XR8SAM, be careful that you compare apple-to-apples.
Offset is related to wheel width as it is measured as the offset relative to the centre of the wheel. What is usually kept constant, to avoid interference with brake and suspension components, is the wheel backspacing. Backspacing is half the wheel width plus offset (refer diag below).
A 6", 7" or 8" wheel may all be suitable for say a Falcon as they have the same backspacing but they will all have different offsets. Sorry don't know that for an AU either but apparently AU offset is 38-40mm geater than pre-AU. Hence the need for spacers to fit factory AU rims to E-series or earlier.