Some paperback editions have a front cover which is a centimetre or two shorter than the actual book. I first noticed this on House of Leaves, where I thought that it's just part of the whole experimental deal (representing the house whose inside is larger than the outside, and/or just putting the book firmly into uncanny valley), but since then I've seen a couple of other books which do this, like this paperback edition of Ken Liu's The Paper Menagerie:
So I'm assuming this is actually a thing that is done for some books, and googling confirms that. However, I haven't found any good sources on why this is done.