People have a +-50% difference in gut absorbed calories. This means that if you take two reasonable outliers, you can have one person who absorbs 200% more calories from the same meal. That's only one factor. Then you need to factor in metabolism. And the body is designed to protect itself from too much fat loss. It is very normal that if a heavily overweight person tries to lose weight, their body thinks it's starving, lowers the metabolism, and starts converting more incoming calories into fat to replenish the lost reserves.
This barrier to entry can make losing weight very difficult as your body literally thinks it's starving, makes you feel lethargic, and can interfere with your life until it eventually re-normalizes some many months later.