The slopes of the small triangles are not the same. 2/5 versus 3/8. This means the hypotenuse of the overall non-triangle is not straight. In particular, the orange triangle has a lower slope than the blue one. Switching them changes the overall non-triangle from having a slight bulge to having a slight indentation. This gives a difference in area which, by a further slight distortion of the grid, allows for a new cell. Nice one, thanks.