There's a very easy explanation, (well two):

1. The commonly known variation of metabolic processes means some people are just more fidgety or have their bodies generating more waste heat. You could reasonably argue this constitutes "calories out", but if it's unconscious, unmeasurable, and varies arbitrarily it doesn't really fit the paradigm of "It's simply a matter of behavioral difference"

2. The part that everyone forgets: human waste contains a non-zero amount of calories, and this varies between people. If your body thinks it doesn't have enough calories (obviously an oversimplification) it will work harder to extract more calories from what you eat. If your body thinks it has enough calories, it'll ignore calories it doesn't want and simply excrete them. The body can tune the amount of calories it gets from food based on its understanding of your environment, and so it's possible to cut down on the amount of calories you ingest and still net more calories from your food than you started with.

Thus, calories in calories out is as much of a fallacy as telling people without money that all they're missing is a simple paradigm of cash in cash out, as if loans can't be adjusted based on the fed's interest rate, their landlord can't decide to charge them more, gas and food can't get more expensive, and they can't lose their job, all outside of their control. This is going to be a poor metaphor, because the human body isn't capable of this, but if a society was super reliant on energy efficiency, tried sticking uranium in a furnace and concluded that it's a poor, useless fuel, they'd be missing that through another process you can get far more energy from uranium. But that's how we look at food: caloric content is measured by literally burning it and measuring how much energy it gives off, as if that's a reasonable analogy for the human body.

I wrote a(n overly) lengthy response to a different reply, but basically, the human body can tune itself to be more or less efficient with its calories depending on how much of a surplus it thinks it has.

The very simple explanation is that human waste has a non-zero calorie content, and can be higher or lower depending on how interested your body is in extracting every last calorie from your food.

This is of course without getting into satiety and the challenges of people society considers too fat being expected to just be hungry all the time, or the actual science showing that dieting is far more harmful for your body than being "overweight", just a very simple explanation for why calories in and calories out aren't as useful of a measure as everyone (including me, by the way) would love for them to be.

That’s a pretty violent straw man you brought to this comment thread.

Do you think it's possible that your net consumption of calories could affect your body weight without being as straightforward or useful of a concept as commonly portrayed?

Why, in a conversation about how valuable metrics are for tracking health, were you so moved as to necessitate using the Holocaust as an example? Do you think that was an appropriate comparison?

Does what I said make you angry? Is it important to you to shock me, to grab my attention at any cost, in order to make your point? Is this how you would respond to someone you perceive being wrong about other points, or is it specific to this topic?

I'm genuinely curious to hear your answers