Keep in mind that the chronological registry system was not devised until the first season of Star Trek: The Next Generation. If I remember correctly, the registry numbers for USS Grissom (NCC-638) and USS Excelsior (NX-2000) were chosen by Bill George at ILM as was the registry number for USS Reliant (NCC-1864). Also remember that the chronological registry system is broken due to miscommunications, or lack of communication, between the art department and the model builders. As far as USS Pegasus is concerned, it is just easiest to accept that she is an Oberth-class starship because it was a re-use of the USS Grissom model.
That's not entirely true; however the bulk of my starship construction timeline is for TNG-era ships, so it's not that big a deal. But I understand what you're saying (see next quote). While that may be true, that only reinforces my opinion that the Excelsior was given a big number because she was a big ship, and conversely the Grissom was given a small number because she was a small ship. Yes, I'm very aware of that, as examples from the FC ships and the Prometheus have shown. Well, yeah, I have to accept that, don't I, because that's what was shown. It just throws a monkey wrench into things, and if there will ever be a remastered TNG, I would hope a new design would be built for it. (Or are you referring to Timo's conjecture that it might be a different class because what we didn't see behind that rock might be different than the Oberth class?...which I don't agree with anyway)