I'm not sure if you are interpreting the results correctly.
Firstly, they say that 29 books were found.
And that 29 were found each of "Fiction & Literature" and "Mystery & Suspense". And 28 of them also were in "Nonfiction". Which means that only one of them is not considered to be Nonfiction. It looks like "The Cat Who Turned On & Off" is the one that is not Nonfiction.
I drilled down into the Nonfiction, and was amused to see that the subcategories the books are in are:
- Nonfiction > Home & Garden > Pets > Cats
- Nonfiction > Science & Nature > Nature > Pets
- Nonfiction > Science & Nature > Pets > Cats
The other categories are just as much fun as under "Fiction & Literature", they are all "Action Suspense", 28 are "Crime" and 27 are "Humorous". So, you aren't allowed to laugh for two of them.
This is not completely Kobo's fault. These categories are set by the publisher, author or whoever it is that adds the books to the store. They set the metadata when the book is uploaded. If Kobo is at fault, it because they have let someone put a book under "Nonfiction" and "Fiction & Literature" as top level categories. Of course, maybe there are cats out there solving mysteries. Unlike mine that just like to jump on the sheets while I change the bed. Or complain when we aren't paying attention to them. Or ignore us off when we try to pay attention to them.