Add your own shipping rates for predefined weight and origin, destination. This configured shipping rates are used to show rates on customer shipping calculator and fetching rates while adding new shipments. In order to featch accurate shipping rates make sure the values you choose to configure rates are matching with the values selected at the shipment creationn form.
The shipping rate calculator on the customer dashboard provide results based on the rate configured for the given origin, destination and weight. If the volumetric weight more the actual shipment weight specified by the customer then the rates will be shown for volumetric weight instead of actual weight.