Perhaps the Constellation (NCC-1017) was the one refit from an earlier type of Constitution, serving as a testbed for components that would eventually become the Connies we all know and love in TOS. Following that, based on an earlier discussion here, the Enterprise (NCC-1701) become the one "freshly built" Connie of her area to be the new "refit" for the testbed of next-generation parts that would eventually build production-run batch of Connie's like the replacement Enterprise (NCC-1701-A), until the entire fleet of all Connies, old and new, were deprecated by the Excelsior class.
Edit: Whoops - looks like C.E. Evans
wrote pretty much the same thing at the exact time that I did.