I'm not seeing any aberrant distortion. I think what you're seeing is the slight bend common to all wide angle lenses due to the convex nature of the front element and the amount of curvature in the 24-70mm is relatively small when compared to all other lenses. In addition, this curvature is even more pronounced when you take your image from a low point and your subject is at a high point. The only way to avoid this curvature is to use a tilt-shift. One of these days I'm going to own a tilt-shift lens...fingers crossed!