BMI was not invented by the Weather Channel.
It's a statistical tool used to group people approximately according to how much weight they're carrying around on a frame of a particular height. It makes no difference between large and small framed people, between muscle and fat.
If your BMI is above 25, you are a member of a group of people who are more likely to get certain chronic health problems. (Yes, a lot of very muscular people are above BMI 25. The statistics do not break these people out separately and there's no way of knowing if carrying weight in the form of muscle could be detrimental over time.) If your BMI is below 18.5 you are a member of a group of people who are more likely to die than the general population.
And if your BMI is between 18.5 and 25, you're probably A-OK as far as chronic health problems but if you want to be in *good* shape you should get some different numbers to work from besides that very simple equation.