You might have made a tiny mistake. Screens rarely if ever depict images as undistorted. Might want to double check using the files pixels to account for your screen distorting or stretching the visual aspect of the image. Such a tiny offset in angle could very well be from distortions.

It’s also drawn on paper and it might not be perfectly photographed from a top view angle. For that you should try and digitally extrapolate the circle to see if it makes a perfect one where the radius is the same on both axis.

Sincerely, a digital artist trying to chime into your geometry project!