I suppose it depends on the p-value threshold you assign to "miracle" status. Where did you estimate his probability of survival? As stated elsewhere, 25 out of 105 people known to have tried something like this are known to have survived.

The wheel well isn't out in the slipstream and probably stays significantly warmer than the ambient temperature, both from having a human in there to heat it and having warm aircraft components nearby. After takeoff, the tires will be hot from hysteresis losses, and the hydraulic fluid will be hot. I'm not sure what other equipment is up in the wheel well for easy maintenance access, but I wouldn't be surprised if the wheel well had several pieces of equipment providing heat throughout the flight. There's also plain heat conduction from the aircraft cabin into the wheel well. The cold also helps survival in hypoxic conditions. Clearly it's not a good idea, but it has been survived many times before.

Edit: http://cdn-www.airliners.net/aviation-photos/photos/1/8/8/17... shows a lot of the equipment in the wheel well, which would have helped keep the well warmer than ambient temperatures.

Read any of these. Especially the first one.

https://news.ycombinator.com/item?id=7626878

Well the article mentions there is a 76% death rate for these known stowaways so it isn't like he's the only one who survived.

My extremely uninformed hypothesis is the mammalian diving reflex kicks in since the article mentioned it was cold?????

http://en.wikipedia.org/wiki/Mammalian_diving_reflex