We should remember that the Yorktown suggestion was based primarily on the fact that the original name of the Enterprise in concept treatises was Yorktown (back when Kirk, er, Pike, was called Winter.) It's an in-joke. And yes, there's no "right" answer unless a canon production intends to address it. My personal take is that the E-A was built from leftover parts from the Constitution refit cycle, similar to how the Space Shuttle Endeavour was built. This would explain why a new ship of an older design would be built while its successor was undergoing trials. I also subscribe to the notion that a new Enterprise was intended to be built as an Excelsior-class ship, but problems with the transwarp drive and Kirk's heroics foiled this plan.