Yeah, this is a topic that has been discussed at length on earlier threads.

A fairly high proportion of gear manufacturers list weights lower than actual in their advertisements; probably in an effort to capture some of the light weight gear market. Some fudge a little and claim that it is just manufacturing variation; others fudge a lot and don't claim much at all. I bought a pack from a major maker that weighed a pound more than advertised. The response from the manufacturer was basically "sorry about that" .

Some manufacturers that are pretty accurate, in my experience, are: Marmot, Tarptent, Six Moons, Big Agnes, GoLite and MontBell. I'm sure that others on this forum can add to the above list. For the rest, take your scales with you when you go to the store.