^Yeah, the whole practice of reusing a registration number with a letter after it is not, by any stretch of the imagination, a normal one. Frankly it doesn't make a damn bit of sense to use it at all. The purpose of a registration number is different from the purpose of a name; a number should go with a specific, unique ship, not with a particular name assigned to different ships. In fact, I think there are cases in real life where the same ship has had its number changed after a refit or on a new mission while keeping the same name.
So there's no sensible reason why every ship named Enterprise
should be associated with the specific number NCC-1701. That's just a silly conceit that the makers of The Voyage Home
threw in for nostalgia's sake and that the makers of TNG unfortunately followed suit with. In-universe, it was supposedly done to pay tribute to the great accomplishments of Kirk and his Enterprise
crew, so it was meant to be specific to that ship and its successors -- though there have been occasional references to the practice being used with other ship names in the 24th century, mainly in the tie-ins. (And when the Sao Paulo
was renamed Defiant
to replace the original one near the end of DS9, it got the same registry number without a letter appended, but that was just because the producers wanted to save money by reusing the existing digital model and stock footage of the Defiant
.) So there's no reason why any prior
ships named Enterprise
would reuse the same registry number with letters appended.