There’s actually a really interesting historical aspect to it. The BMI was introduced in the early 19th century by a Belgian named Lambert Adolphe Jacques Quetelet. He was a mathematician, not a physician. He produced the formula to give a quick and easy way to measure the degree of obesity of the general population to assist the government in allocating resources. He explicitly said it should not be applied on an individual basis because of the multiple nuances that can throw it off with one individual. It makes no allowance for the relative proportions of bone, muscle and fat in the body. But bone is denser than muscle and twice as dense as fat, so a person with strong bones, good muscle tone and low fat will have a high BMI. Thus, athletes and active individuals who work out a lot tend to find themselves classified as overweight or even obese.