You read a book and might think that the name on the cover is the name of the person who wrote the book, from the first word till the last word. The writer who wrote it all. The writer managed to come up with all the ideas, the structure, the plot etcetera.
If you want to believe, there is no problem, just keep on reading the book you are reading.
There is a mysterious person in the book world. It is an editor. This editor looks at manuscripts, by longstanding writers, by aspiring writers. The editor makes comments, proposes improvements, suggests to skip this or that passage, or even both passages. The editor looks at the continuity of the story.
In this article you read more about the mysterious editors.