What Mark does is a variation of a ketogenic diet. High in fat, mid in protein, EXTREMELY low in carbs. Your body typically looks to two sources of energy: Fat, and carbs. Normally, your body uses carbs and converts them into glucose, which runs through your body. This is a VERY important process for your fueling brain functions. If you significantly cut down the carbs in your diet, you liver will convert fats unto "ketone bodies" which, in the absence of carbs, replace glucose as an energy source. It's essentially a form of starvation, which teaches your body to cut fat because it does not have any other option for energy.
Often, people who are not in solid shape, and those who are not hardgainers, probably want to stay AWAY from such a diet. Mark's solid physical shape, hyperactive metabolism, and muscle mass allow him to burn fat much quicker than the average person, which is why such a diet would be EXTREMELY beneficial for him. Fat is a much greater source of energy than carbs are, so Mark can actually work out harder, lift heavier, etc. because his body is incredibly efficient at using that fat as an energy source.
I hope that clears up some confusion.

As Taoist mentioned - there are many different "ratios" at play, and a lot of the results you will see are dependent on:
1) Your goals
2) Your body type/weight
