because my Audi has 4x100PCD and the wheels are 4x108PCD. What this means is that the stud fittings on my wheel hub are fixed on a 100mm diameter circle.
ie 4x100 equals 4studs on100mm diameter.
The wheels I have are for a 4x108PCD hub. Same number of studs (4) but greater diameter (108mm). Hence the holes on the wheels are are slightly wider apart. Great for later coupes but not for mine.
This website explains it nicely
http://www.carbibles.com/tyre_bible_pg2.html