Comment by userbinator

Due to the addition and subtraction, errors introduced in the x step are effectively canceled out in the y step, and vice-versa. This is also the reason why the path is slightly elliptical --- at some points there is slightly too much y and not enough x, at other points not enough y and too much x, and the other two combinations (for the other two quadrants) too. However, all the errors cancel out so the path goes back to where it started.