Imagine looking at a set of x-y-z axes, from directly along the z-axis. Say in this image, the road is the line z=0. Why is it, that only lines on or parallel to the z-axis get skewed? All the surfaces that are in or parallel to the x-y plane remain undistorted, and their angles are at 90 degrees. Just to highlight the point, imagine if this image was extended to include more building and streets at the side. The streets at the edges would get increasingly more skewed but again, only in the z-axis. What is it that is causing this increase in skewing?