Why your Novel changes

Most novels go through many rewrites and after the writer gets past the point where he or she does not make many grammatical mistakes, it comes to the point where the character takes over or the author understands that the character would not do that. Most start using their own experiences and their own personality. So the character is a reflection of you. If you are immature, the character becomes immature, if you hate mussels, the character also hates mussels. If you love to sew, the character is the son or daughter of a weaver. And this leads to the character sometimes doing things that you did not intend to or what makes him look stupid.
Another thing is taking out necessary conversation. One needs not ask how the weather is on the planet Xerion or shake hands with all the relatives, if it is not going to advance the story. So if your ship can only land on Xerion in calm weather or your hero is a spy, and that is the code phrase, you do not need to have that conversation. Consequently unless shaking hands with relatives is a way to discover that one of your relatives was replaced by a serial killer and that serial killer forgot that your aunt had a prominant wart in her right palm, it is no need to go through the "hi Bill," et cetera.
One that seems to be is making a preview, a story before the main story that gives the back ground of the story.
Another thing is practicality. If you plan for the hero to get back by Christmas at Vancouver, and you had your character be stuck in the LaGuardia airport on Christmas Eve, then you had better change his home location to Rochester, New York rather than Vancouver, Washington.

Now these are reasons why often the novel at the last draft is not the same as when it started. Usually when you first start, you record all the conversations instead of the ones necessary for the plot. You also have the characters talk long monologues without taking a breath ——at least i do, and have a character suddenly appear as by magic, or leave and then he is in the scene the next chapter, when he is up north somewhere or dead. Then there is the way his name is spelled.

By the time, the last drafts appear, you are finally getting things right and then you start cutting out unnecessary words and cutting out the extra sentences, and finding that the butler's name is not Hey you. By then, the fun starts and now you have to look for an agent, or if you have lots of money, publish it yourself.