Manufacturers may be not wish to publish figures as they are concerned about not meeting targets and the consequential contractual liabilities.
Such would relate to a specified build state. Even then, the figures will vary. I spent some years looking after a squadron of nominally identical aircraft .. while the equipment list etc confirmed that, the weights/CGs gave the lie to the fiction.
The weight may change (hopefully reduce) during the life of the product
Rarely reduces. Invariably, in normal service, the empty weight increases, due mods, repairs, general dirt and rubbish and so forth. One usually only sees a reduction of note associated with a significant mod program (eg change a bunch of ancient valve kit to you beaut electronic stuff or, more commonly, a good clean out). I recall a DC3 I used to look after many years ago - had a real good clean out during a major and shed several hundred pounds in the process
The weight of airline specified cabin fittings may vary so much that it makes a single published figure meaningless
For a fairly common fleet, one might use fleet average weight data to save some dollars on reweighs. However, usually it is preferable just to run with the individual hull data to minimise any required ballast fudging and, in consequence, maximise useful (ie dollar producing) payload.