I'm not sure anyone else is going to get back to you on this. I mean this site is really only oriented to bodybuilding; which normally consists of keeping things as simple as possible. EAT, LIFT HEAVY, EAT, SLEEP. In other words you may be better off going to the library and reading up on the subject because it is a little bit complex for the bodybuilding forum to say the least. Who knows maybe that just me. With that being said, here is what I can tell you...
METABOLISM: "the sum of chemical reactions happening in your body. Allowing cells to grow and reproduce, maintain their structure, and respond to their environments." [Wikipedia]
To put it simply genetics play a big role in how well this process works. From what I know, that's just the way it is. Blame your parents [as they say]. On top of your genetic make-up you can increase it slightly by adjusting your body composition.
The breakdown of body composition goes like this:
Muscle Tissue contributes to approx. 20%
Fat contributes to approx. 5%
Organs contribute to about 80%
Other then that I can't tell you much more about it. So to answer your question(s), I don't think it really has anything to do with what the person in question is doing, but the fact that they have good genetics. Their body composition will only play a small role in all of this.