I think you answered your own question regarding use of France and also didn't take into account the destruction of the dry dock at St Nazaire, but this answer would only make sense if the area could still be used for the deployment of U-boats. It doesn't sound as if this is the case but I'm not familiar with the game. Anyway, sounds to me as though it is intended to reflect strategic considerations rather than the historical facts of precise dates when territories changed hands.