It’s not the weight got by weighing a box on a scale. The volume weight is the space occupied by a box in the carrier. Thus, your price will be based on the volumetric weight of the boxes rather than the actual weight.
To understand better, take a cushion or a teddy bear, weigh on a scale –both are lightweight, but by nature are bulky. Carriers will not charge these commodities as per the weighing scale, as their volumetric weight is often on the higher side.
While packing, make sure that the box is not bulging out from any side. Always use a right size box to pack your items. If you have bulky things, like a quilt, pillow, teddy bear, etc, vacuum pack them - this will save a lot of space. Do not leave any loose space inside the box - so it doesn’t swell in transit.