>I've always believed in the law of thermodynamics myself - counting just the calories
Counting just the calories ignores second order effect, and diet is all about second order effects.
If one could absolutely stick to counting the calories and eating the specified amount, of course one would lose weight.
But e.g. a diet of only having 1500 calories of broccoli for 1 year, and one would be ill from lack of tons of nutrients and vitamins (and they'd have stopped much earlier anyway).
E.g. if one eats certain kinds of foods, your appetite increases and it's more likely to overeat. Not giving one's self some foods they like, also makes it easier and more tempting to break a diet. And lots of other factors (exercise, hydration, nutrition, and so on) unrelated with counting calories, that still affect what we eat and how we process it.
Any disparity is just on this single sentence: "your appetite increases" (as opposed to "their" or "one's").
"Their" as used across the comment is not meant as the plural possessive. It's instead used as a gender-neutral pronoun (a common use) to avoid repeating "one's" or using "his/hers".
It, in "your appetite increases and it's more likely to overeat" etc., refers to the situation/possibility (as in "If they sky's cloudy it's more likely to rain"). It is normal to use different pronouns to refer to different entities.
Finally, the "we" at the end is perfectly fitting. It's a generalization, beyond what each individual does.
Though "they" was used as a gender neutral pronoun -- not to change the subject from "one" to some group. E.g. as in: "if one drinks milk, their bones would benefit".
Counting just the calories ignores second order effect, and diet is all about second order effects.
If one could absolutely stick to counting the calories and eating the specified amount, of course one would lose weight.
But e.g. a diet of only having 1500 calories of broccoli for 1 year, and one would be ill from lack of tons of nutrients and vitamins (and they'd have stopped much earlier anyway).
E.g. if one eats certain kinds of foods, your appetite increases and it's more likely to overeat. Not giving one's self some foods they like, also makes it easier and more tempting to break a diet. And lots of other factors (exercise, hydration, nutrition, and so on) unrelated with counting calories, that still affect what we eat and how we process it.