True. My grandparents were farmers, but I never was in the buying/selling end of it. I always understood it to be a volume measurement. I even looked it up, because I wasn't sure, and it came back as a volume measurement. Then I went to figure how they accounted for different products, because I knew a bushel of apples weighed differently from a bushel of corn or wheat, and found each product was given an accepted weight per bushel.
Kind of a really weird way to get to the value.
Because produce presents problems of both weight and volume, keeping bushels by weight is a good system.
If you go to an orchard and pay to pick your own, you will likely pay by volume. However, when that same orchard sells to a wholesaler by the truckload it makes far more sense to sell by weight.
Also, grain and produce can vary a great deal on size. Thus the amount of empty space in a bushel basket would vary greatly. A bushel basket of large kernels would make less wheat than a bushel basket of small kernels.