Or, hey, maybe the fact that there's a HUGE HOLE through the middle of the ship means that in fact, a very large portion of the ship's volume actually *is* air .
The volumetric figures are for habitable volume, hence they don't include things like the flight deck which is one giant space in the middle of the hull.
As an aside the flightdeck's volume for the Victory is about 1.8 million cubic meters.