Two big reasons and one small one yet important, muscle maturity plays a large role but maximizing your food is the main key. Don't buy into what those idiots on the G&O say, learning how to both maximize and manipulate both your carb and fat intake will help a lot.
Another important note, if your body is beaten down it doesn't matter how many carbs you eat, you're not going to fill out as well as a rested body would.
All of these things are important, unfortunately there isn't a key formula to follow as it will vary from person to person based on not only genetics but present physical state. Nevertheless, the principles apply to all.