No exposition is provided in any episodes as to why a refit before the refit was provided. The only relevant dialogue is in this vignette:
"This experimental engine design, if successful, is slated to be used in all future refits--Enterprise's included. Command wants them tested close to home."
I'm a big fan of not explaining stuff like that. Filling in the unsaid, interstitial stuff is too often done in fan efforts to the neglect of more essential aspects of stories.