The long side (hypotenuse lol, gotta love those dictionaries) of the triangle (the big one, with all four elements) isn't exactly a straight line (compare the sides of the red and dark green triangle, they don't have the same proportion). As you can see when you look good, in the picture above the 'hypotenuse' is slightly dented and in the picture on the bottom it sticks out a bit, that's why there's an open square all of a sudden .