Model kits make decks flat with no camber because it simplifies construction. The curvature of decks is subtle enough that it is easy to overlook an error on a model when they are flat. Changing the decks to have camber can be challenging to a new model builder. A flat plywood false deck can be stiff and not want to conform to cambered beams, and must be pre-bent by soaking it in water and letting it dry over a curved form. The curved form should have slightly more curve than the desired curvature, since the plywood will spring back a bit when removed from the form. Getting the curve close and not perfect is all you need. Glue and clamps or pins or weights will make sure it is final-formed and firmly attached to the beams.
The amount of camber does vary slightly between ships, and the lower decks, ones more likely to be wetted, have more camber than those much higher, such and the quarter and poup decks. Generally the camber should be around 1/4" for every 12" of beam, or about 2cm for every meter of beam.
The mahogany and birch plywood for this upper false deck on this model has only a slight camber, but that is all the curve you need to properly shape the deck, which is ready to accept a layer of planks. The original kit beams were replaced with curved ones to support the new shape of the deck.
The more you research wooden ship design through books and forums, the more you find how kits have made shortcuts in the accuracy of features for the purpose of simplification. This is why the beginning ship modeler finds more and more of these inaccuracies, as they gather more and more information. Those who want more accuracy out of a kit will make changes and modifications, and the "bash" kits into models which have much more detail than as built right out of the box, and they will discover a new level of fun from doing so, increasing their skill with each carefully though out change.
Often times, these changes involve rework or complete replacement of parts and sections of the hull by scratch building. Once you figure out the scope and method of how you want to make a change, scratch building small changes is no longer feared and confidence in your work is built along with the model. Then you realize with some experience is that to make improvements,, all you need do is buy some more wood and maybe a new tool or two.