Earlier quoted context omitted.
I don't eat particularly healthy, and I don't exercise much, yet I'm perpetually borderline underweight. Why does the math not matter for me, while the next person watches every calorie, goes on long walks, and yet struggles to lose any weight?
Because you're not eating enough? You can eat only McDonalds and still lose weight if you're eating below your daily resting calories. Your body composition will probably change for the worse though. Likewise the person who struggles to lose weight with diet and exercise is very likely eating more than they think they're eating (or opting for healthy options that aren't actually healthy, like salad covered in 600 cal…
I used to eat at a salad bar every day for years. I got to observe what all kinds of people put into a “salad”.
My salads: spinach, romaine, broccoli, carrots, cauliflower, chick peas, mushrooms, balsamic vinegar (no oil).
Obese people’s salad: Four hard boiled eggs, ham, three leaves of lettuce, shredded cheese, a cup of ranch dressing.