Just like one calorie does not equal another calorie, losing 3,500 calories does not equal to losing a pound.
A calorie of broccoli is not the same as a calorie of potato chips. The calorie of broccoli is more nutritious; it has more vitamins and minerals and nutrients than the calorie of potato chips.
You can survive longer on broccoli than you can on potato chips.
It used to be thought that in order to lose a pound of fat, all you had to do was to cut out 3,500 calories from your diet. Conventional wisdom was that if you cut back 500 calories per day for 1 week you would lose a pound.
Turns out that is an extreme simplification of weight loss.