A lean, heavily muscled athlete can score "overweight" or even "obese" on BMI despite having very low body fat — the formula only ever sees total mass and height, never composition.
BMI was built by a 19th-century mathematician studying population averages, not individual bodies, using nothing more than mass divided by height squared. It correlates reasonably well with body fat across a whole population, but for any one person carrying unusual amounts of muscle, it simply has no way to know.
It fails in the other direction too, and more quietly. Someone with low muscle mass and high body fat can sit in the middle of the healthy range while carrying the risk profile the measure is meant to flag — a false reassurance nobody investigates, unlike an athlete's false alarm.
Estimate with the rule, then check it against the calculator that models it properly.
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