"Let's turn it around -- why would a manufacturer knowingly maintain the *wrong* product specs on their website?"
Easy. Let's take a hypothetical vendor that gets products made from factories in China. Actually, that is pretty likely. Let's assume that the vendor is trying to cater to the ultralightweight crowd (us, here). If most of the vendors of the same kind of product list the weight as 1000 grams, and if our hypothetical vendor starts selling them from a website that lists 1000 grams, then we understand that. Two months later, suppose they list "new and improved" and 950 grams for the same thing. If questioned about this, they can claim that they simply started getting the products from a different factory, and that may be true or false, because buyers have no right to know exactly which factory they came from. If questioned more about this, they could claim that the advertised weight is actually +/- 5%. Well, that hides a multitude of sins. In some cases, a low advertised weight will increase product attractiveness.
All I can say to the consumer is to study the vendors, study the product reviews, study the reported weights from other consumers, and then make your own decisions about one vendor versus another.
Another game you can play is this. Contact a small cottage manufacturer and ask about how many of Item XYZ are in stock. Then ask if he will go through the stock and "bin" them according to weight. Get him to pick out one that weighs only 930 grams instead of the one that weighs 1030 grams. Some will do that for you. The big companies will not. Geez, REI doesn't even list the weight of most products.