This is my take on the problem.
First the coal mine basic was first introduced in TRS2004 time frame. We did not have bethgons with the load capacity of the ones we have today. So loading a 100 to 150 car consist was not a problem.
The built in coal hoppers have the load capacity set at 54,300. One of the RRmods coal hoppers has the load set at 127,992 . So now with the RRmods hoppers 1 car equals 2 older hoppers.
The coal mine basic does not have the capacity to load the larger hoppers without either missing one or half loading one.
Hope this helps explain it some.
Kenny