GUI feedback display

I control the laser machine tool through Ethercat. When I interpolate and draw a circle, because of the driver gain setting problem, the interpolation circle is not round.
like this:
 

But it shows a very round image on my image. I am using an incremental encoder.
like this:
 

Can anyone explain me the reason? I think it’s a problem with the drawing algorithm

Sorry, my question is "Why is the circle interpolated by the laser an ellipse but the circle shown in the image is a perfect circle?"