If you throw a brick in water, When the ripple starts off it will be oblong. In one direction it will be about 6 inches by 3 inches. 1 second later the ripples will have expanded by 3 feet so the total ripple will be 6 foot 6 inches by 6 foot 3 inches. After 2 seconds, 12 foot 6 inches by 12foot 3 inches etc.

So basically it is never perfectly circular, but the errors from a circle stay the same size as the ripples expand rapidly.