I think you'll find the higher volume listed on the NSP website includes the displacement volume of the dugout area - hence the reference to "reserve volume" in the marketing description.
As a comparison, a 25" wide Starboard Ace would have a volume of close to 400 litres if the dugout area was included in their volume description.
Perhaps Starboard leave it out to stop people losing their mind.
Don't be alarmed, at the end of the day the board doesn't have too much volume - it's just the design.
And it works - hence why there are so many dugouts around now from other manufacturers eg. DEEP, ONE, NSP, DC, plus Starboard.
I know it's not always possible but the best thing is to try a board before we dismiss it. You'll surprise yourself on what you can paddle.
As a reference I am 6'2" 105kg and regularly paddle a 25" Ace comfortably for racing, flatwater training and DWing. It sometimes has it's challenges but the rewards far outweigh the disadvantages.
I wouldn't hesitate to paddle one of the dugouts from the other brands - they look and go awesome!