The revenue you gain depends on how many pages are read in your books. There are also bonuses for authors who gain the most page reads. Therefore, some unscrupulous authors engage in unethical practices, such as book stuffing and inserting links to send people to the end of the book with one click of the link. Amazon's official policy is to ban these people, but enforcement of the rules have been lax at best. David Gaughran has written an excellent overview of the Kindle Unlimited book stuffing problem, which is highly recommended reading.