Builder’s models and drawings frequently had differences to the finished product as design details frequently changed during the construction process and drawings were sometimes updated, and sometimes weren’t. From this process drawings would be copied and distributed at varying points along the design and build process so invariably including different numbers of the modifications.
From that point on the modifications continue throughout the ship’s life, some being included in original drawings as later versions but frequently only existing as a modification drawing.
One of the more extreme cases, and one to which I have always had close dealings with, would be the QE2. Every major dry dock throughout her career included significant changes that would completely change the appearance of a model of her. You basically have to choose a specific point in time and stick with it. It would depend on which aspects of her appearance you thought you like and those you didn’t. The biggest external visual differences would be the likes of the Magradome, the Penthouses, the Funnel after the re-engining, or even the pebble grey hull in 1983!
Imagine a model of the SS Norway, who in their right mind would want to include the two landing craft on the fore castle!