Hello 3image,

at the end every circle is represented as a nurb curve in modern CAD. The number of control points isn't a criteria to decide if it is exact or not. The magic here is the knot vector. You can find a very good desciption for that on page 9 in the following document: http://digiitalarchfab.com/portal/wp-content/uploads/2011/03/Nurbs-Curve-A-Guide-for-the-Uninitiated.pdf

Have a nice day
Karsten