A human being has to respond "Yes," "No," or "Partial Quantity" to the request (e.g., if you want 4 copies of a book but they only have 2 on-hand). BolianAuthor says he got a "Yes" back, which means someone took the time to give a wrong reply to a book they either couldn't locate, or couldn't be arsed to locate. Regardless of whether the on-hand quantity showing in the system was correct, a human, not a computer, sent him an incorrect response.
Once the error was revealed, the proper procedure would then be to have a manager manually update the on-hand quantity in the computer so this wouldn't happen again.