[pwdisswordfishy]

Best bet is a user/user-education error, a crummy implementation and most instances involving an ISBN-10 printed on a label with a UPC nearby; the ISBN-10, being most likely printed in OCR-A, needs no barcode in order to be decoded, and the barcode being read by the scanner app is the one corresponding to the UPC.

Scanning the "ISBN bar code" then is actually leading to a lookup for digits of the UPC (a totally different sequence).

It's also the case that unless you were a used bookstore, you're fundamentally dealing with different kinds of inputs than someone cataloging a home library.