I had this same question in mind, i.e. how are you establishing a weight discrepancy from published ratings without knowing the curb weight?
The door stickers are pretty accurate in payload as they are delivered to the dealer, so you can easily figure the curb weight from that (GVWR - payload). The large discrepancies people see are from the owners manual vs actual sticker. This comes from dealer added options as well as model configurations.
The owners manual can't specify weights for all models and options as the configurations are infinite. Therefore they list the base model as a guideline. Usually a regular cab 2x4.
I'll use my '14 Ram 2500 as an example. The owners manual shows weights for 3 trim levels with the 5.7l engine and 6 speed but doesn't specify anything else. It shows max payload of 4,410lbs. I have a crew cab 4x4 which has reduced that to 2,340lbs as per the door sticker. That's over 2k difference due to the added weight of the larger cab, extra seats, 2 more doors, 4 wheel drive.....