Look at this
ARC(-2000, 1000, 2000, -1000, 0,360);
OVAL(-2000, 1000, 2000, -1000);
After runnig script enter zoom about 50000% ..
These objects are not geometrically identical.
I belive that math behind an oval object is true ellipse formula, but the ARC object defined by values entered above is just asymetrically scaled circle ..( try to create circle, make group from it and then change scale of the group using grips along the x axis... the circle inside group will change into closed b-spline resemblant ellipse...)