this is simply Archimedes principle. "An object floating in water displaces its weight in water". It may float a little higher or lower in salt or fresh water (because the water density is different).

Now, manufacturers are notoriously optimistic on their quoted weights. And usually the optimistic weight is effectively empty, before things like batteries, water, ground tackle, sails, stores, spares, dinghy, dodger, etc; so called "light ship" displacement. A small cruising boat can pick up 2 or 3000 lbs without any trouble at all.

Next thing to keep in mind is that travel lift gages are at best a very crude estimate of weight/displacement and could be off by 10 or 20%, if they are truly functional at all.

To determine the real weight/displacement, you need to hoist it on some recently calibrated crane scales, these can be rented near major cities. Lacking that, you can find out from the boat builder where the waterline is supposed to be at "design displacement" (which is likely going to be light ship, but they may have a fully loaded figure in mind as well). If you can get a reference from them that says the water should be X inches down from the rail, stem, chainplate, or something you can get to, then in calm conditions and eyeballing no heel, you can measure to the water and see what your "sinkage" is. If you are below the number they give you (very likely) then you also need to get from them what the water plane area is. This increases with sinkage (because of overhangs and the flare of the topsides), but using the design figure will get you close. Take the water plane, multiply by the sinkage, calculate the additional volume displaced, multiply by around 64 lbs/cu ft, and add that to the design displacement. The factory or naval architect can give you the water plane area in "lbs per inch sinkage" which is the same information expressed differently and saves a little bit of calculation.