Of course it's not. And it never was. All it says is that if you are inside the "normal" range, you're definitely healthier than you would be if you were outside that range.  I have epilepsy, myopia (-6.5/-3.5) and some other problems at a BMI of 20.67. But if I were overweight (or underweight), I'd have to deal with even more problems - and that would just suck.