It is a very misguided principle, look at the way many athletes and sportswomen are built, especially tennis players, swimmers and rubgy players, I doubt many of them who are muscular through their sport have a "normal" BMI but I doubt anyone would accuse them of being overweight, unfit or unhealthy.
Also womanlyness can add extra weight, I weigh more because I have boobs and a bum and, as my mum would put it, child bearing hips, my body is built to support them and I'd never be a waif and if I weighed the same as some of my naturally more petite friends, I would probably be seriously underweight and just look wrong.