The most variable slice of daily burn
NEAT, non-exercise activity thermogenesis, covers all the energy spent on movement that is not deliberate exercise: walking, standing, fidgeting, posture, and the small motions of daily life. Across individuals it is the single most variable component of total expenditure. Classic overfeeding research found that the change in NEAT between people could span roughly 1,500 to 2,000 kcal per day, which largely predicted who resisted fat gain.
Because NEAT is mostly unconscious, it shifts without a person noticing. The same individual can spend hundreds of calories more or fewer in a day depending on mood, sleep, and how restless or sedentary they feel.
Why it scrambles calorie predictions
NEAT tends to fall during a calorie deficit and rise during a surplus, which means it actively works against intentional changes in body weight. This adaptive movement is a major reason a calculated calorie target can mispredict real-world results, and it explains some of the wide variation in how people respond to the same diet.
This is general educational information rather than personalized advice. A qualified professional can help interpret individual energy needs and activity levels.