Many books these days seem to have a lot of extra pages after the end of the main text. These pages are either appendices, bibliographies, author interviews, or previews of other books. When you don't know those features are there, it can appear like there is a lot more story left to read.
So, I love it when those pages throw off my mental timing. It means I'm not "reading" ahead trying to figure out what is next for the characters in a book. When I'm in to a story, my mind can wander about to try to think of where the plot is heading. The longer the book, the more like I am to come up with plot trajectories in my head. It can be distracting. Books ending sooner than planned stops that mind wandering.
It also means the unexpected endings gives me a rare moment to pause and think, "Hmmm? What am I in the mood for now." I often plan on what I'm reading next when I know a book is ending. When I don't get the chance to do that, my mind runs wile with possibilities. Abrupt endings give my brain a break and allow for transition time from one book to the next.
These moments are rare, but it means I love it even more when they happen.