In my mind geological barriers trump all imaginary lines. Latitudes means little when there's 25 kilometers of water between you and the other side of land. Counties have irregular shapes mostly due to geographic features making it historically difficult to easily traverse over the areas that would become boundaries between two counties; cultural differences between these counties are a phenomenon that arises because of on-the-ground geography rather than imaginary latitudinal lines and to me, that's why they take precedence.