The kits that I have built generally list a weight range. For example, 6 1/2 lbs to 7 1/2 lbs. My goal is generaly to build the model lighter than the bottom number. Sometimes I can. Generally my models land on the bottom number.
I have never seen or built a kit that couldn't be built as light as the specified weight on the box. But then again, I haven't built many new kits. Most of my kits are designs that are 20 or 30 years old.
So, perhaps this is a relatively new trend by kit manufacturers to list optomistic finished weights. I have purchased some modern ARF models that advvertised a flying weight of 4 1/2 lbs, a fantastic number for a 40 size aerobat. When I got the ARF, the box actually listed 5 1/2 lbs. That was a disappointment.