http://rimowa.de/downloads/katalog/K...12-2013_EN.pdf
Tey say it lots of different places, for lots of different models. I agree it seems a bit iffy but I'm sure they've measured!
I can only assume the wheels are indented differently and I can see the zip is placed more symmetrically around the 'equator' for the spinner. The body of the case also sits differently between the wheels, but it's really not clear where that extra couple of litres is coming from.
Nice to have it, though.