Adapting Tolkien's works for film is unusual in that normally all you have to adapt is the book itself. The book is one lens into the "story" and the adaptation is attempting another. A good adaptation attempts to be a new lens on the story underlying the book rather than the book itself.
But in the case of Tolkien, the story is already fleshed out more than the book because of the rest of the legendarium. In the case of The Hobbit, this is particularly the case.
That's not to say Jackson hasn't messed with the story but it must be remembered that, as I've said above, this is a re-telling of the story underlying The Hobbit more than an adaptation of the book itself.