Why is it that when books are turn in to movies, they are so different in the book or they tie in other books in the series and make a mess of things. For example: Anne of Green Gables, Anne of Avonlea, Anne of Green Gables: THe continuing story

Some things that work well on the printed page would fail on the silver screen, and vice versa. For example, a lot of Harry Potter and Lord of the Rings fans were upset that the movies were not exactly like the books. But if they had been, the movies would have failed because it's a different medium and the artistic rules regarding what works & what doesn't work are different. The Lord of the Rings, for example, has passages of extended dialogue and other passages that would have been excruciatingly slow on the screen. Peter Jackson used artistic license in deciding what to use, what to leave out, and what to add translating the story from one medium to another.

I understand that part. I don't care if a movie is exactly like the book but when the movie is based on a book and it is completely different

Because what makes a good movie and what makes a good book are very different. In book time is not an issue you can put the book down and read it again, plus its all words and you using your mind to exstaperlate what people, things, and places look like.
In movies the audience is using their site, it is a visual media, you see what the people look like, there is music, and well the whole shanbang is different. You have to convey the same story, that in the book may take two months to read, in two hours. Thus you have to remove things to make it a better movie. The fact is movie maker are not making a carbon copy of the book, they are telling the same story in a different way, it is impossible to make a movie that the book is the script, if people did that, the movies would stink, plain and simple. Now sometimes the book is better than the movie, and the movie is better than the book, some books can't be made into good movies.
Though I do not know the movies you are talking about, it is highly probable the director, and the screen writer, felt it nessasary to make the changes they did. Maybe they were not good changes, but remember they were trying to make a good MOVIE, not a visual copy of the book.