To answer your question as to why the glass bends. Glass naturally has a slight distortion near its edges and depending on the smoothness of the surface, it can distort a lot of what is being shown. This is not an incorrect functionality, but an accurate representation of a natural glass material. One way to reduce the bend a bit is to decrease the thickness of brush, but this won’t do much.
Also be sure you have some sphere reflection captures on either side of your glass to capture the floor and the scene. See the image I have posted as a reference. When you move the spheres around you can see how and where the floor reflecting image will be shown on the glass. Thanks and I hope this helped clarify your question.