The way that the cargo capacity is measured is very subjective. The Q5 is 540 litres = 19.1 cubic feet according to this website:
http://www.parkers.co.uk/cars/specs/...px?deriv=45068
The X3 is 550 litres according to the UK brochure = 19.4 cubic feet.
Both figures with seats up obviously.
I think the differences are to do with the size of the number of virtual boxes you can fit in or something like that. It's not pure volume.