WorldCat.org by default shows the holdings for all records in the FRBRized workset. The "Find a copy in the library" section displays symbols for all libraries represented for all items in the workset. To see library ownership information for a specific edition, click the "View all editions and formats" link.
You can double-check whether a library holds an exact edition from the FRBRized workset by selecting the "Just this Edition" link above the list of holding libraries.
It is also possible that the item has been withdrawn from your collection, but your staff have not yet updated WorldCat with this information.