By now the OP knows why you go to the scales. They cut through all of the BS and give you the weight at that moment.
Worrying about 100 or so lbs is foolish, you can shift that much with 15 lbs of water accumulated in the fresh, gray and black tanks. It is less than the weight of a tank of gas on the TV.
Also, was the TT was actually weighted or estimated from the construction details. Who knows? As a rough rule of thumb, assume the UVW does not include propane, battery options and such.
Finally when referencing the CAT scales. In general it means the Semi scales that give you the weight on each axle. One trip without the TT and one trip with the TT gives you the axle weights so you can see the effect of the WD hitch and the total towing package at that moment.