Volume in surfboards plays an important roll. Volume helps buoyancy flotation for paddling, keeps the board high through turns which means less drag, helps boards carry the board through flat sections. There does seem to be too much emphasis on the "volume" number, and its use on determining the right volume for the individual surfer. There are so many other design variables that over ride this one aspect of design and it just numbs people into just thinking about a number that "fits" them, and gives them confidence into buying a surfboard online. It gives the consumer something tangible and a number is familiar.
Whats more important regarding volume, is how it is distributed. The balance of foam from nose to tail, and how it foils, rail volume and deck roll. These play an important roll in determining surfboards performance in all aspects. If you have design flaws in these areas then the right volume isn't going to help you.
More focus should be put back into design.