My theory on registry numbers is a bit complex, but I'll try to make it as plain as possible:
1) I posit that the NCC number represents a "hull number" (I like the old Naval Construction Contract idea). Each ship is assigned a unique hull number at the time the contract is made. The ship ordinarily will keep that number throughout it's operational life, even after significant refit (which explains registries like Constellation
2) Not all ships of a single class or type have contiguous registries. Aside from refits (Constellation
again), special case exceptions (Enterprise
[DS9]), the production run of a given ship type may be broken up into "blocks" or "flights". Starfleet might issue contracts for 10 Constitution
(refit) class heavy cruisers in 2272, for example, then in 2282 issue 10 more. The later 10 hulls would have the hull numbers for the next 10 available "slots" in the construction schedule, whatever those numbers might be.
3) Contract numbers proceed in ascending order from the inauguration of Starfleet (Federation version). The first Federation starship hull was NCC-01, and the numbers rose from there.
A side note about potential discrepancies, such as Excelsior
, who was depicted as being a "new" ship in 2285 despite having a registry lower than those known to be in use over a decade earlier (such as Entente
's2120). I submit that the "Great Experiment" spent all that time as a "hangar queen" as the Corps of Engineers tinkered with her transwarp drive. The reactions of Kirk and Scotty in particular suggest that they were well aware of the ship, and had a certain amount of amused disdain towards it. This would be consistent with a ship that the "yard birds" had been promising would be "ready any time now" for quite some time.