I would assume because lifting the back of a hive and estimating the weight, or hooking a deer scale on it, is a WHOLE LOT EASIER and QUICKER than going into each box of a hive and pulling and counting frames with capped honey.
I've just purchased a 100lb hand held spring loaded scale just for this purpose. The style I chose is the Chatillon IN-Series Mechanical Scales. The main selling point was the max weight indication.
I have always taken for granted that if I support the front of the hive and slightly lift the rear the reading is half the total weight. That value minus the weight of similar empty equipment gives a close approximation of the stores inside the hive.
Even if I'm incorrect, I'm pretty sure it's close enough for government work.