Here's another possible reason why the antenna design would improve reception.
The human body is a conductor capable of taking on and releasing electromagnetic force.
The antenna for the new iPhone is on the outside of the phone, and thus can be directly touched by you.
At the right frequency and wave length, the antenna can actually pass on its pulse to your body, essentially turning your big dumb head (and the rest of your body) into an extended antenna.
This only works with certain waves, but it is definitely possible that this could occur.
Again I'm not saying this is actually what happens with the new phone, I'm just throwing some science at you, and letting you know that it's a possibility.