BMI is a VERY general measurement. It's only height/weight, and that weight is not just fat, but muscle and bone as well. BMI was developed not for individuals, but to show population trends overall. A lean but well-muscled individual might show up as "overweight" or even "obese" according to BMI, while a less well-muscled and more fat-laden person can be "normal weight" according to BMI.

Much better measurements are either waist/height ratio or waist/hip ratio. That is much more likely to show overweight, AND it's showing the intra-abdominal fat that is most likely to bring health issues.