I think one variable is that some people's lungs are more efficient at processing oxygen than others. IIRC, aside from the doping, Lance Armstrong's lungs could process something like 30% more oxygen in a certain amount of time than an average male of his same size. Just good genetics.
An article I read way back when, about successful Everest climbers, stated that the older climbers processed oxygen better than those younger,. I don't know where the age cut off was, but on also has to think about muscle mass, and strength, which also determine success.
