Are the shape changes accurate, as you drag north/south? I would expect horizontal borders (like the north border of the USA, which is along the 49th parallel)

Are the shape changes accurate, as you drag north/south? I would expect horizontal borders (like the north border of the USA, which is along the 49th parallel) to stay horizontal as you drag. But in the tool the border becomes more concave as you drag it further south. Why is this?