I can see both sides of this problem.
From the manufacturer's standpoint, they have to deal with the fact that the marketing materials and packaging were probably produced before the gizmo actually went into production, as well as completely independently. If aforementioned gizmo is actually produced by a factory in China or Vietnam they probably won't know the *actual* weight until they receive a shipping container full of them.
Add that to the fact that I suspect that there might be quite a bit of weight variation between "batches" of a product it is hard to know what weight they are supposed to put on a product. And given that early production runs were probably made by a different factory than the big production runs probably makes things even more confusing.
I also strongly suspect that any last-minute design changes, especially ones that either make a product more durable or easier to manufacture, increase the weight.
Until pretty recently few gear-buyers actually bothered to weigh their purchases anyway, and that is probably why most manufacturers don't really know what to do when you call them on it.
On the other hand, if you bought company Y's gizmo over company X's based on weight, and the delivered product from company Y weighed more than company X's advertised weight, you certainly have a right to complain to company Y.
Probably the best way to deal with this problem is to vote with your money, and buy gear from companies that stick pretty close to the published weight for their products. At the same time, naming and shaming companies with outrageous variations in the marketed weight and actual weight is also very helpful.