[GeoffR]

An ePub book can have a number of different identifiers, some publishers use the ISBN as the identifier, more common seems to be to use a UUID, but they can use anything they want or even leave it out altogether, there is no consistency at all.

When an ePub is sideloaded the Kobo firmware grabs whatever identifier is in the book and displays that in the ISBN field of the book info page. It doesn't check whether it really is an ISBN or not, it just takes whatever the publisher has given as the identifier.

If the book has multiple identifiers then the firmware takes the first one (or is it the last one?) and others are ignored. So sometimes a book might have a valid ISBN identifier but it isn't used because another identifier is present.

There are only really two ways to ensure that the book's actual ISBN is displayed in the ISBN field of the book info page on the device:

1. Edit the book before sideloading to add the ISBN to the OPF file as an identifier and remove any other identifiers.

2. Or, after sideloading, change the device's database entry for the book to use the real ISBN. Easiest way to do this is with the "Update metadata in device library" function of the KoboUtilities plugin in Calibre (assuming you have entered the real ISBN into Calibre.)

(Edit: I should add: If you sync the book direct from Kobo then the ISBN displayed on the device should be the same as what is on the Kobo website because it is added during sync rather than read from the book itself, but if you sideload the same book then the identifier in the book is used, and it won't necessarily match the website. There is nothing to stop the publisher using one ISBN on the website and a different identifier in the book itself, and many do.)