I think the following is correct:-
While looking for data on a topic in this forum,I noticed that the GRO index "master list" of districts contains the two variants of the title.Looking at the master listing Carnarvon applies up to 1937,Caernarvon from 1937 to 1974(local government reorganisation?),and slightly confusingly Caernarfon(Caernarvon) from 1974.
It would be easy looking for an early record to start typing "Cae..",accept the predictive text and not find records which are under the Carnarvon variant.
Regards
Roger