The final cut of a movie is often not the directors cut. The director gives the studio their cut, and then studio executives get to do what they want. Good execs with good directors know not to fuck with a good thing. But if the movie the director hands in stinks or if some bad audience previews get execs scared all kinds of things can happen
Basically, this is why the idea of a directors cut even exists. Because what you see is rarely what the director handed in when they were done.