Actually, the type of truck is driven just by nominal load capacity and not the max GRL, though the max GRL's are what is referenced when determining truck size, which is why I said to add those up to determine. Why? A railroader can correct me, but from my understanding, the max GRL is used because it has always been a required formula. The CAPY line would make more sense, except history has it that it wasn't derived from a standard formula, making it meaningless across different car types, and the requirement for it was eventually dropped. (You'll see its absent on some modern cars.)
I see the math from your POV, but I am not entirely sure why the light weight is not included for determining the truck capacity. You'd think the car's weight would need to be included or maybe its because there is some AAR or FRA rule that requires a car to not be over a certain light weight, which is somehow silently acknowledged by truck designers already.
In the below example, the CAPY is 53T and the max GRL is 220K, so this matches up with 70T trucks.
And yes much of these larger boxcars were used for appliance and/or auto parts.
I can only imagine that as time moved along, some of these cars were shopped and upgraded for heavier loads, since 110T trucks (286,000 GRL) are the standard today.