I work at a library, and there are books in the catalog that don't exist

I have been working in the cataloging department at my library for a little less than two years. We use a database program to manage our holdings, and each item in our collection has a unique catalog record with all the normal library catalog stuff like title, author, subject, and so on. I have noticed that some books are cataloged, but the library owns no copy. I can see all copies in the catalog, and these books have no item record. It is not strange for libraries to have a record in the catalog for items they don't own, but use or borrow, but these are books we have never owned or borrowed. I have noticed that a lot of these are university press books, and they are from about the 1950s to the 1980s. I have never seen an actual copy of these in person. I have never noticed a request for them from a borrower, and it has never caused any glitches in the catalog. I have asked people who have been working at the library longer than I have if they know why these are in the catalog, but they don't know. So, I was just wondering if anyone else has noticed this at their library, if it is normal in libraries, and what their purpose is.