Okay, I'll bite. It is true that air cools faster than water, because of its lower heat capacity. However for the air comprising the headspace to cool, it must first lose it's heat through the bottle and this is also true of the water below. I just think the reason it freezes first at the top is that the water/air interface and film on the inside of the cap, provides a better place for ice nucleation than the body of the water below. We are not talking about ice forming below and floating up because that isn't what happens.