He also points out that there are many ISBNs that are “wrong”, but are actually correct in the real world. This is because publishers don’t always understand about the checksum and just increment the ISBN when publishing a new book. In many library systems there is this checkbox next to the ISBN entry field where you can say something like “I understand this ISBN is wrong, but it is correct in the real world”.

So just flagging wrong ISBNs would lead to a lot of false positives and would need specific structures to deal with that.