All "ratings" are SWAGs (Scientific Wild @$$ Guesses) created by engineers, based on:
1. The manufacturer's weight limits of tires, wheels, axles, springs, etc. Keep in mind that all those ratings, except tires, are "hidden ratings". The vehicle buyer does not usually know what they are, and the vehicle manufacturer often places a rating on, say, the axles, that is less than the axle manufacturer's rating.
2 What the marketing department wants. Have you ever noticed that Gross Combined Weight Ratings on trucks will go up every year, even though not much has changed other than cosmetic items? BUT, the ratings will seldom exceed those that might require the driver to have a CDL! Yeah, it is a "numbers game". How can a truck that has a 2000 lb. payload be rated as a Half Ton or a Three Quarter Ton?
Long ago and far away, we were full-timing in a 32 foot triple slide fifth wheel pulled by a 1994 Dodge 3500 CTD 4X2 standard cab long box dually, with 5 speed manual transmission, 4.10 rear axle ratio, and Load Range E tires, registered for 14k GVW. The GCWR of the truck was 18k, IIRC. When I finally got around to weighing the rig (on the final trip back to MT before settling down again), it was under all ratings except GCWR. The total combined weight was 21,180 lbs. That's right, it was a little more than a TON AND A HALF over GCWR. Yet, we had made several trips over the Continental Divide (Homestake Pass, Lookout Pass), over the Cascade Range (Snoqulamie Pass) and over the Sierras (Donner Pass) with no problems. As I am over-fond of saying, "Nothing bent, nothing broke, nobody was injured or died!"
It is worthy of note that the GCWR of a vehicle is usually found only in the Owners Manual! It is not one of the ratings found on the door jamb sticker. All of the numbers are "ratings", not "do not exceed" limits.
In those states where the registered owner can buy the GVW he wants (like the 14K GVW on the '94 Dodge 3500), it IS a "do not exceed" limit!
NOTE: The above is my opinions, and a description of what I have done. It is not to be construed as a recommendation of what others should do. Any decisions others may make are purely THEIR decisions!