I think much like the Sovereign class version, it was built under a different name then changed based on events.
I never understood why Starfleet would have other Excelsior class ships under construction when they still hadn't proven that the prototype was workable?
Look at the differences between the 1701-B and the NX/NCC-2000. There's a good chance that both ships started life as testbeds for different/competing drive systems. Which would make them both being built in or around the same time not to unusual.