Inevitable revisions are the main reason I prefer not to predict the release date of a specific novel. As most authors know, the writing process can go along smoothly for quite awhile and then all of a sudden a mental wrench is thrown into the works. It is not writer's block per say, but rather a new direction which can develop over the passage of time. In other words, the author can sometimes be inspired by new ideas that only become feasible as the storyline unfolds and ultimately progresses in their imagination.
Not to worry, though. I've been writing this way for most of my life, so I'm accustomed to the pattern.