Why a discrepancy between sticker payload & actual weight?

Some mfg's do not include everything when they do their numbers. For some you have to ad the weight of things like the spare tire, tow hitch, jack and even the rear bumper. You also have to ad in your weight the weight of your cap fuel and everything you have in the truck. Since your truck is a 2012 it may not even have the original tires if the new ones are bigger or higher load rating there will be more weight there too.