My hypothesis is that the equation that weight loss is related to calorie intake minus expenditure, but I think the conclusions one draws from it fail to take into account how much the body can control expenditure.
My body seems to naturally drift up in fat weight, very slowly -- probably around a pound per year -- no matter how much I eat. If I cut calories a lot, I can lose fat. After that, my fat level drifts back up to the old target weight pretty quickly, and then up slowly again. I'm pretty sure if I plotted my fat from the time I became an adult, it would be a pretty straight line, with short-term dips and recoveries for diets.
Losing weight sustainably would mean resetting that target somehow. I don't know how. Changing my diet and exercise levels don't seem to do much for fat levels (but do more for my general fitness and health).