It's mostly about net calories, for managing weight loss or gain. That's what most people are interested in. A forty day fast is gonna deplete fat reserves, simple as that.

When I hop on an exercise bike and burn through a few hundred calories, that's great. Then if I turn around and go eat a 900 calorie (I'll be talking in terms of the SI "kilo-calorie" unit, like on food labels) meal, what good was it, if I could have eaten a 600 calorie meal that was just as filling? I wasted my time doing cardio. OK, not exactly, your cardiovascular system would be the more healthier for it, but your weight wouldn't change much.

To put things into perspective, a pound of fat is around 3500 calories. Burning 10 calories per minute on the exercise bike is a brutal pace, and that's 350 minutes. So you've got six hours or more per week at the gym vs. altering your diet to eat 500 calories below your BMR each day. One seems markedly easier to me than the other. The trick is getting all your micronutrients in on a restricted diet.

To finally answer the question; 70% is a reductionist estimation that merely serves to emphasize how altering your diet will probably be the easiest way to see results. Even in the case of trying to gain muscle (or any mass), I would again try to relate the importance of diet.