A common way for manufacturers to determine their pack size is to use polystyrene balls/beans to fill the pack then transfer those into a box and measure.
From that the volume is rounded off to the nearest common size.
(not always as there are 548L and so on packs...)
Most manufacturers add the real or imagined external pockets volume, I say imagined because often you can't fill those pockets if the main compartment is full already.
(check photos of the full pack in Google Images )
Do make sure that you have water/food and fuel included in the volume required.