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.
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.