While it's always best for a movie to be planned out to the letter and to have an organised and efficient shoot, the most important part of the filmmaking process is easily the edit.
Good editing can mask a continuity gaffe, diminish the impact of a bad performance, or even turn a meandering scene into a rapid-fire sequence of punchy one-liners. Because of how rewarding editing can be, it's no surprise that filmmakers love tinkering with their projects for months on end - so much so that they can occasionally continue to tinker even after their movies have been screened to the public.
Maybe they want to change a blink-and-you'll-miss-it moment that doesn't really matter in the grand scheme of things, or maybe they want to chop huge chunks of footage from their movie after receiving a poor reaction to the first cut. Or, maybe it's not even up to the director, and some sort of outside controversy or internal conflict requires certain edits to be made.
Whatever the case - while these post-release changes didn't significantly alter the films in question - it's fun to think that a lot of people out there saw the original, untouched versions of these movies, before they were taken back to the edit bay for a final snip.