It is mentioned on the device page: https://doc.e.foundation/devices/herolte/
I think that U is for US SIM-unlocked model, so it is not really relevant to sell it on Fnac (but maybe it was clearly mentioned?).
One has to be very careful with this: I bought a refurbished S7 (on BackMarket) and I received twice a US model from resellers based in France or Europe. I could send them back without any problem, because the model mentioned on the page from which I bought them was the F model (and they finally send me one).
If it was not clear on the Fnac website, maybe they can accept to replace it or to give you a refund, as you wanted a Europe/International/Non-US model? Also check which processor was mentioned on the page.