The advertised weights are supposed to be "ready to fly"

You'll never get a great number from any manufacturer or mail order web site because none of them list the specific items used to complete the model.

Maybe a reviewer will post an actual weight on a finished model, but they do not always do that. If it's something you're interested in it is helpful because they usually tell you the components they used.

I'll pick on Great Planes and Tower. I had a GP Big Stik 60. The advertised weight is 6.5 pounds. Mine weighed 7.5 pounds.

They recommend .61 to .91 two stroke or .91 to 1.2 four stroke.

Maybe the prototype used a 65LA. I used a 91 Surpass. There's a several ounce spread between the two engines.
I used Hitec Dual BB servos for all control surfaces. Non-BB servos would have worked fine and saved weight.
I replaced the foam wheels with rubber wheels.
I installed a slightly larger fuel tank.
I converted it to tail dragger.

Did my changes add a pound? Or, was the wood in my ARF slightly more dense than the prototype? I have no clue.