I already mentioned many ways were a given product can end-up with multiple GTIN codes: a book with an ISBN and a UPC, a book with two national ISBN codes, or products with two product codes on the same box. There is yet another reason for having more than one GTIN: relabelling.
One of the goals of the whole GTIN system is to avoid the need of adding labels to the boxed product: the code is on the box and serves as primary key to be looked-up. Of course this only works if there is no price on the box, or if the price does not change.
I have at home a tower of hanoi game produced by Éditions Trédaniel which has an ISBN printed on the box,
9782849331231, along with a price, 22€. The code and the price were covered up with a label with a different price, 16.90€, but also a different ISBN:
9782849332566. While it makes sense to cover the price, why change the code?
I’m not sure, but this product is in some ways a book: there is a booklet inside and it bears a book number (ISBN), which means that it might fall under the Lang Law, which lets the publisher fix the price of a book, which is printed on it, the seller must respect this price, he is only allowed to give a 5% discount. This also means that if a publisher wants to lower the price, he needs to change the label, but also probably the code. As here both ISBN fall into the same range, this is probably what happened.