Normal window pane glass is fairly straight and does not change the angle of the rays after they pass through the glass. The light may change direction when it enters the glass, but it will change back to the original angle when it exits the other side. Therefore, no effect to the light that you see coming through it. Your glass on the other hand is convex and concave in nature, with the distinct requirement that light will be affected as it travels through. Since the light changes, even if just a little bit, the area behind the glass will have a different level of illumination than beside it. We will see that as a shadow.