It probably has more to do with antenna response characteristics due to polarization (antenna is similar to a j dipole http://en.wikipedia.org/wiki/Slim_Jim_(antenna) or a modified inverted V (since it's a U) maybe http://en.wikipedia.org/wiki/Inverted_vee_antenna ) + isolation from your hand. Without your hand (which is similar to a non-efficient ground plane) screwing up the antenna response, a vertical iphone would likely have better reception, especially if it's a dipole-like.