Some may knowingly publish false information. Some aren't paying attention -- perhaps marketing & production don't communicate. Others use semantics to obscure actual field weight (e.g. trail weight = tent + pole only).

What's equally as problematic, is that many of us backpackers believe what we read and hear, and without further investigation, carry those erroneous specs to fellow backpackers as if they were gospel (hey, I'm as guilty as anyone, maybe moreso).

Remember, intermingled with all the good stuff, there's a lot of inaccurate information floating around backcountry chat rooms and newsgroups. Some people just don't do their homework. Anyway, beware !

I've found published gear weights to be consistently understated - anywhere from a couple ounces to over 1.5 pounds. Occasionally, they are accurate, rarely are they overstated - that would be evidence of poor marketing practices :-) 

Examples:

In the past, I've purchased about ten Dana Design packs, including five terraplanes. The Terraplanes all were 3 to 7 ounces understated. The spec for the most recent one was 6lbs 9oz, but it weighed 6lbs 15oz (a 6 ounce understatement).

The Mountainsmith Wizard was advertised at 3lb 3oz, but it was really 4lbs 12oz ! The 1996 Mountainsmith Mountainlight 4000 weighed 2lb 5oz. The 1997 design weighed-in at 2lb 10oz without tramp pocket attached, but was still advertised at 2lb 5oz. (Note: This is true in 1999 of the Vortex STX packs and many others that change designs but do not update their specs.)

Another popular ploy is to publish tent weights sans stakes, sacks, zipper ties, seamseal, groundsheet, and so on.  The Outside Online Gear Guy and others will tell you that the Garuda Jalan Jalan is a primo lightweight tent at 3lb 6oz (that's what it said on page 8 of the 1997 catalog).  However, I say it's a primo lightweight tent that weighs approx 3lb 6oz + 2 to 4 oz for stakes + 2 to 3 oz for seamseal + zipper pulls, guycords, parachute cord for tieing out wing doors, etc.  Bottom line - actual "dry" field weight for the Jalan is 4 lbs.  Oh yeah, almost forgot, want a groundsheet ? Add another 6 to 8 oz.  Don't believe me ? - look on the back page of the Garuda catalog !

Another popular mind teaser - trail weight. Sierra Designs - and others - advertise their tents using something called trail weight, which is tent + pole (excluding everything else). For example, they advertise the Divine Lightening as 2lb 2oz and, of course, that's the weight everyone - including retailers - seems to be using.  Mine weighs 2lbs 12oz.  All I added was two ounces of meticulously applied SeamGrip, zipper pulls, elastic cord door tie-out, a 15 foot strand of Triptease LightLine (practically weightless guyline), two Easton aluminum stakes, and 2 very thin/light nylon stuff sacks.  In addition, I removed a 2 ounce SD advertising patch from the tent and do not use a groundsheet.  Yet total trail weight is 2lbs 12oz.  If weight is an issue with you, Be Careful !

SD advertised their Summit as 5lbs without vestibule and 6lbs with it. Backpacker Magazine (Dec 97) weighed it as 5lbs 7oz without and 6lbs 9oz with. Again, in both cases, that's only tent and pole - no seamseal, stakes, etc.

That's one nice thing about the Backpacker Mag's gear reviews - they publish their own weights in opposition to the manufacturer's specs.

Anyways, there are many more examples, but the bottom line is:  don't take manufacturer's specs seriously. If weight is important to you, take the time to actually weigh the stuff. Most all good outdoor shops have a scale. If not, buy the thing, take it down to the post office and weigh it. If you're not happy with the results, take it back. 

Also, before you tell someone else what a piece of gear weighs, weigh it yourself or include a disclaimer (e.g., manufacturer's stated weight).