Sometimes it's thanks to a surprisingly strong car, sometime's it was just luck. There are some crashes where you just can't believe someone would walk away from the wreckage, but they do.
Who could ever believe that Robert Kubica would walk away from this 186 mile an hour crash at the Canadian Grand Prix in 2007. He got a sprained ankle and a mild concussion. The first thing he said to the emergency responders was, "Will I be able to race in Indianapolis next Sunday?"