That's because the bag is not an empty shell. Anything inside takes up packing capacity. Pockets, dividers, straps, thickness of the bag shell, etc. . In some cases, the wheels are included in the measurments.
To get the liter capacity, the standard test is to fill the bag with small pellets. Then the total amount of pellets is measured and you get the capacity. It's not just the outside measurments.