I don't know, my reaction is "wow, Americans truly love reading," or alternatively, "they really like to boast their numbers even in anonymous surveys."
Even if you only read fiction, 8 books are like 500k to 100k words. That's a lot of words!
Series is really what makes the number so high IMO. I read a lot of fanasy/sci-fi which is often a lot of trilogies. Reading just one trilogy puts you above the median. I have several friends that read only 3-4 books last year, but several that also read as much or more than me. Discussing the books amongst friends helps, as we recommend books to each other. Book-tok and other book-centric social-media circles are huge.
And it may seem like a lot but that was spread across an entire year. I often read a few chapters before bed each night, but it often depends on how hooked I am on the book, I make more time for it when I'm more hooked on a book, or on a deadline to return the book to the library.
Audiobooks helps carry the number higher as well. Its a lot easier to "read" a book when you can do it while doing other things. Although I prefer to sit down and dedicate time for e-books, I do listen to some audiobooks as well, and many of my friends exclusively read via audiobooks.
It's starting to sound like reading books has a similar distribution as consuming cannabis. The "daily" users account for 3/4ths of the spending on cannabis despite being 1/3rd of overall users. The heavy consumers consume A LOT and skew the statistics.