[Ogedei]

How can it be more complicated at the base level. Your body burns X calories to survive. If you eat less than X your body uses it's stores to make up the difference, ie muscle/fat. If you eat more than X your body stores the excess as muscle/fat incase one day you eat below X.

Your body is not above thermodynamics. Energy can neither be created nor destroyed, only change form.

All of the hard gainers I have met have been extremely active people. Extremely active people burn a lot of calories and don't have time to consume much, so it's harder for them to meet their calorie demands, let alone add some on top.