The "virtual volume" concept was used for a few seasons when boards became wider. It was used to estimate how much volume a narrow board would have to have in order to give the same feeling of float.
The fact that some boards have less, or more, volume than indicated by its model number comes from first deciding what models there should be in a line, and then developing them. And it seems that in some cases the initial guestimates did not really match the actual board that became approved for prodution. Thus, no "virtual volume" anymore...