That's not really how I interpret it at least. I think its assumed there is no one with a perfectly efficient digestive system, and that there is variance in efficiency based on a plethora of factors. However, that efficiency rate hardly matters. You are in effect a car. Some people are hybrids, super efficient perhaps, while others are SUVs guzzling gas to go the same distance. That being said one thing is true for all of these cars: if you put in 9 gallons instead of 10 gallons into the tank, you will go for less miles than if you put in 10 gallons. In other words, if you are being accurate about measuring your calories in (the gas) and you are measuring your weight (miles driven), you can adjust the input calories to effect your output weight. Exercise then is like driving in low gear with the engine at a high RPM; you are burning more calories in a given amount of time just like you'd be burning more gas in a given distance running at a higher RPM.