While the GTIN system was intended for logistic up until the point of sale, the emergence of marketplaces for second hand goods means that many products, and their codes, appear once again in system. As often with legacy data, this means the old assumptions still affect the new system.
French comic books used to be quite different from US comic books: different format (A4 typically), hard binding, they commonly had ISBN codes. Some popular series were sold in supermarkets, and thus had an EAN code, even before the mapping of ISBN into EAN codes.
One character I loved was Gaston Lagaffe, and I still have a few albums of 80s edition. They are interesting because each album bears an ISBN-10 number (no barcode), but also an EAN code, except all albums of the set have the same code:
5410983209003. The album Gare aux Gaffes du Gars Gonflé has ISBN-10
2-8001-0308-6, while Gala de Gaffes à Gogo has ISBN-10
2-8001-0093-1, both have the same EAN. If I search that code using the Red Laser application, I get another album: Le Cas Lagaffe.
I can only hypothesise on why the publisher chose this scheme, in those days the selection of albums in supermarkets was pretty random, and the price of all of them the same, so it might well be that supermarkets would just handle those albums as minor variants of each others, i.e. the supermarket would buy a selection of them, but consider them all equal for inventory purposes. This is still the case for smaller toys, where items with different colours share the same code.
Of course, each book has an individual ISBN-10, and if I convert it into an ISBN-13 and then look for it, I get the correct results.