In the case of Excelsior, for whatever reason, they decided to convert the prototype to an active duty ship before building NCC-2001, which makes her number not technically correct, but these things happen sometimes in real life.
Or that is the norm? First ship is 2000 and that doesn't change and she's the first ship. First ship is 1700 and she's still the first ship. We're not given any indication that the Excelsior had an unusual naming convention or classification.