PLARAIL Thomas and Friends 75th Aniversary Clear Thomas Island of Sodor set pre-order

parcels are not shipped purely on weight . The couriers use volumetric weight . ie The cost of a shipment can be affected by the amount of space that it occupies on an aircraft, rather than the actual weight.
 How to Calculate [b]Volumetric Weight[/b]

1. Multiply the parcel's three dimensions (length x height x width)
   
2. If you are using centimetres then divide the result by 5000 (or 4000 for EU economy services). ...
   
3. The resulting figure represents the volumetric weight of your parcel.
   

So using the 5000 constant, Volumetric Weight is exactly the same as Volume measured in units of one fifth of a litre. Why is the word "weight" involved?