I have been obsessively reading and logging manufacturer specs and weighing the actual items for 2 decades. I used to lug a big balance beam baby scale and tiny postage scale everywhere until I upgraded to digital. There are stores that when I walk into w/o a scale under my arm they actually say, "Dude,where's your scale?". I have spent countless hours annoying engineers, product designers, marketing types and presidents with questions and suggestions about the weight of their products. With that background in mind, I offer the following.
There is usually a significant variation in weight between examples of the same item due to small differences in size, fabric thickness, wall thickness, hem size, amont of extra fabric not trimmed etc. I dropped the weight of my North Face tent 8 oz.! by pulling out 15 tents from the back room at my local outlet store and combined the lightest of the 15 canopys with the lightest inner and the lightest poles. This even happens with high end manufacturers like a 4oz overweight 2lb. Western Mountaineering sleeping bag I weighed (and did not buy). I am happy to say that lately I've purchased a number of products that have weighed in spot-on:14oz Go-Lite Cave-1 tent, 14oz RAB Quantum Top Bag, 19oz The North Face Beeline 900 sleeping bag. But these are still the exceptions. I still consider any spec that doesn't come from Dr.J's scale @BPL or mine to be highly suspect.
The following probably relates more directly to your question then anything I've said so far. I learned this from Eastern Sierra legend Doug Robinson when I orded a "lightweight" 3-1/4lb internal frame backpack that he designed for Montbell 15 years ago when no one knew of this company making 7oz,2 layer un-lined GTX bivy sacks and 12oz GTX jackets.
Marketers invent specs for products and tell the design team to meet these specs. The designers discover these specs are impossible to meet so the marketers respond to this information by publishing these specs to help sell lots of products regardless of the realities imposed by physics and the time-space continuum.
Another situation is that the actual specs of the prototypes are published in catalogs, press releases, dealer information packets and hang tags to meet publication and manufacturing deadlines. Then the actual production manufacturing materials, components, and techniques (which can vary greatly from prototyping) are utilized to produce the often heavier product that the consumer sees in the store or catalog. By then it is too late to publish corrected specs and frankly, many companies see no reason to do so. The only way to discourage this practice is an army of dilligent, educated consumers with scales under their arms. Doug was able to get ahold of a pre-production prototype for me that almost met specs. In fairness to Doug, the paragraph about marketeers is my observation/inferrence. The comments about prototype Vs production weights are from him.