ok, first this forum is dedicated to the American definition of backpacking (which is wondering around the woods with a shelter) and not the European definition of backpacking (budget travel). You seem to be more interested in the later.

Either way, I think there are people here who can answer your question. There are no standards on how a company rates the size of there backpacks. As a result you can get much variation from brand to brand. As to why a longer backpack would hold less, that can be as a result of the other two dimension (width and depth). Backpacks are really tricky things to buy and the most important thing is how they fit on your body. It is really best to go try them out in person. See how much room they have and how comfortable they are on you when loaded up. If you can't try them out at a local shop then focus on companies with a hassle free return policy. Order a couple you think will work and return the ones that don't.