Then you have the USDA weight bushel.
USDA Weights, Measures, and Conversion Factors for Agricultural Commodities and Their Products identifies a bushel as equal to 4 pecks or 2150.42 cubic inches since it a volumetric measurement.  Since it is far easier and quicker to weigh a load than measure the volume of the load, the USDA created standard weights for grains so they could be bought and sold by weight and converted to volume using USDA established Individual commodity weights and measures vs trying to accurately measure the volume; it's not a weigh bushel, rather a standardized conversion factor.  Test weight concept was developed to account for varying densities of grain and since lower density takes more volume to store and transport which increased storage and handling costs so the industry began docking to offset the increased costs.