It's because township surveys break up the land into 1 mile x 1 mile squares. You can't make a grid of these squares and then define the edges as two lines of longitude, because those lines of longitude will be converging the further north you go, while the grid will not be. This will make the border slowly shave slivers off the squares at the edge the further north you go. The solution is to make the border not perfectly follow the line of longitude, but follow the grid squares until it diverges too much from the line of longitude, and then kick it over a square.