Why the estimate and reality diverge
TDEE calculators combine a resting-energy equation with an activity category. Both parts describe population averages. Your movement outside exercise, training efficiency, body composition, food logging and short-term water changes can all move the real number.
Use a range, then observe
Choose a reasonable estimate and keep your routine steady long enough to see a trend. Compare weekly averages rather than reacting to one weigh-in. Sodium, carbohydrate intake, menstrual-cycle changes, travel and hard training can change scale weight without representing the same change in body tissue.
Adjust the model, not your self-worth
If the trend repeatedly differs from the estimate, the estimate needs calibration. It does not mean that your metabolism is “broken.” Large or unexplained weight changes, restrictive intake, pregnancy, an eating-disorder history or symptoms that concern you deserve qualified professional support rather than more calculator adjustments.