Hence my crazy fan theory that the NX-01 was never mothballed, but used by the Federation shipyards as a testbed for new technologies and configurations. New engines, sensors, internal configurations, hull elements, computer systems, etc almost on yearly basis. Finally they hit so successful formula that the ship was used as model for the Constitution class and recommissioned to active service as the NCC-1701. But every now and then, when the tech guys wanted to try something new, she was recalled to dock.
After a century of tech trials, fits and refits, the ship had such a jumble of legacy systems and redundancies that made it extremely efficient and durable, but required a Captain level miracle worker as the Chief Engineer. Compared to it, the production line models seemed like they were put together by monkeys.