This is a very simplistic measure. When using this as a measurement for obesity, you must consider each individual. In the case of this girl, the BMI is the worst measure possible. Anyone could look at her and tell she is far from obese. She is a healthy, active, normal girl.
BMI is basically your weight divided by your height. In athletes, the BMI will show them to be obese. Why? because muscle is more dense and weighs more than fat. You can take an athlete that weighs 200 and is 5'10", and someone who is overweight, doesn't exercise and is 200 and 5'10", and they will both be obese by the BMI measurement. That's why BMI must be incorporated into a common sense health screening and cannot be allowed to stand alone.
If insurance companies begin to use this as a standard measurement, we will be categorizing a certain percentage of the population as obese who are far from it.
There are much more accurate tests and measurements to evaluate the percentage of body fat, but they cannot be done as quickly, or some cases, in an office setting. The hydrostatic body fat testing is the most accurate way to measure lean tissue and fat tissue, but it also requires the person to be fully submerged in a tank of water.
Bottom line, if you know you are overweight, the BMI is an indicator of your health. No more no less.
It should never stand alone as a test for the overall health of an individual.