We don't worry about the 'actual volume' of the tank when filled with fish and rocks and substrate and such. Like Paul said it is more of a guideline. The problem is that it isn't really the volume we are worried about but the area and depth (which calculates into volume, yes I know). You want sufficiently strong light spread completely across the area of the bottom of the tank. This is in turn inhibited by the depth (sort of). There is a falloff of available light from a lightbulb as you move away from the lightbulb. The water isn't really deep enough to compound this problem in most of our tanks, it is just a failing of the type of light source.
Aquascape? I'm a crypt farmer.
It's a fine line between fishing and standing on the shore looking like an idiot.
That IS an aquascape, it's titled "The Vacant Lot".