Body mass index (BMI) might not be an accurate indication of body fat and health risks for?
BMI is calculated as follows;

This calculation simply give the feel of how heavy the person is. It doesn't give an exact readout of the fat measurements in the body. There have been studies which shows a correlation with the abnormal BMI and increased abnormalities related to weight gain. Since the method more or less give an arbitrary measure of fat levels and it's non-invasive this is used a crude proxy for fat levels in the body which in turn is used to predict the health risks related to high-fat levels/ obesity. There are scales of BMIs that people follow for this crude measurement but it is not necessary that it holds good since the proportion of fat-weight to bodyweight will vary from individual to individual, in different age groups, different genders, etc.
Body mass index (BMI) might not be an accurate indication of body fat and health risks...