Books are tagged as a whole, meaning a certain tag must be prevalent throughout the work for it to get added to the title. If a title is tagged anal but only has it for a few pages in one chapter, I believe that would set the wrong expectations for many potential customers. If you want to scan a book for a certain, less frequent tag, you can do so by checking the individual chapters once they're all released (~two weeks after pre-order goes live). The same logic applies here, but on a smaller scale, so if one chapter out of ten has NTR, that chapter will have the tag.
A quick way to scan through a title is to go to the artist's page; book chapters will be grouped together, allowing you to do a quick search in your browser. Let's use
Michiking as an example. We recently released a book from them, so all the top results are from that book. A search gives no results for any of the NTR variations, meaning the title is "safe" if you want to avoid that content. Likewise,
sixty-nine only shows up briefly in two out of the book's 14 chapters. Those chapters carries the tag, but the book as a whole doesn't.
I hope I shed some light on our reasoning and gave some tips on how to better evaluate the content of a book.