This refractive error is due to the eye shape being significantly oval in shape such that light is focused unevenly on the retina and blurred vision is at all distances.

Most people do not have perfectly-shaped eyes. Most often it is the cornea (front surface of the eye) that has uneven contours. When the curvature of the astigmatic cornea varies across different directions (like the surface of an egg), then light passing through the cornea will be focused unevenly and the result is an asymmetric blur.

With different amounts of blur in different directions, objects may appear distorted or even doubled. Some letters may be easier to read than others depending on the orientation of the lines in the letters.