We don't all use food and fat at the same levels of efficiency.
No we do not. Nor have I, nor anyone else, suggested that we do. But that has nothing to do with what I wrote.
X and Y are variables.
X = Calories put into your body
Y = Calories burned by your body
X > Y = You gain weight
There is no debating that. Some people have much higher Ys than others, but if your body uses 1200 calories a day on average and you eat 1100 calories YOU MUST LOSE WEIGHT. There is no independant place calories can come from. Your body cannot spontaneously create them. You can't get them from sunlight. You MUST consume them.
Find a picture of lean times in history, lets say pictures of Confederate soldiers in 1865. Find the fat one. You can't, of course. You think none of those guys had low metabolisms? Of course they did, but the environment surrounding them kept them from consuming even that low level of calories, and presumably also required them to raise their Y value by moving around. It is not genetics or luck that tells us there were 450lb-ers on the front lines at Appomattox Station, it's that those people were hungry...