Warning* Opinionated context, do not read, for you may explode.
could it have to do with where the flats were going. Assuming that they were not all destined for the same location, although all looking the same they very well could be. If you have more than one car's worth of load, no point in loading one full and the other at 20%. Even out the load. Also, possibility of crossing lines into areas of lighter locos. Maybe the train will have to be broken up, and there in lies the same problem, no point in having one set of cars loaded, and the others partially full.
just my $.02, - training, - sales tax, - state tax, - convenience fee, - handling charge... Expect a bill...
Casey