You may have a problem with your Barcode reader, and that it is not reading slot 24 correctly, but, the behavior that you describe is pretty normal. The Barcode scan jist reads the Barcodes, and compares that to a list of Barcodes that it has in the Internal Database
A device scan will load the tape, read the header file, unload the tape, and update the IDB and the GUI as to what is found, in other words, much more complete than a Barcode scan