Body Mass Index is a screening tool — weight relative to height squared — not a diagnosis. It is fast, free, and surprisingly good at the population level. But at the individual level it has well-known blind spots: it cannot distinguish muscle from fat, and it says nothing about where fat is distributed.
A rugby player at 5% body fat can register as "obese"; a sedentary person with low muscle mass can register "normal" while carrying unhealthy visceral fat. That is why clinicians pair BMI with waist circumference, waist-to-hip ratio, and body-fat percentage.
A practical rule: if your BMI is 18.5–24.9, waist measurement under half your height, and you are physically active, BMI is doing its job as a rough indicator. If any of those fail, dig deeper with body-fat and waist-to-height tools before drawing conclusions.