Tonnage/Capacity

"Now, I'd wonder if the volume of coal is such that the total cubic capacity of the car is reached before the weight capacity?"
Charlie, that may explain it as well as it can be.
I've always understood the max. tonnage ratings are to indicate tonnage of cars so the overall tonnage of a train can be calculated against the ruling grade, which includes the weight of the car itself.
From eyeballing models of a DB Gon relative to the tender of a K-28, which as indicated with boards is 9 tons, there's no way more than two tenders could be filled with one load.
You are right anyway, cubic volume is not relative to weight in any way. The "Capy" ratings are probably related to ICC billing for CL shipments, and irrelevant to actual car weight.
I know there are others who know a lot more about this than I do, maybe we'll hear from someone.