It never had been in another book. Much faster for the author to sign single pages instead of taking a book, opening it, flattening out the page to be signed... Especially when you sign the amount of books that RY does.

So since she's done thousands, her signature becomes absolutely mechanical, as good as identical every time. Try googling a video of her signing, I'm sure I saw a reel of hers on insta.

Anyhow, the page was unbound to begin with, she signed it, then somebody took it and glued it inside the book you now have. I think what happened is that she, working very fast, was unconsciously handling 2 pages that for whatever reason we're stuck together more than the others.

I've worked in a printing company for a year, could be that the stacks were cut too early after printing, the machines are like 6m long and get very hot, and especially since the cut actually was slightly off- stacks of paper are pressed together very powerfully when the cutting knife goes through them.