Dont take BMI seriously. Ill try and keep this nice and simple.
BMI takes your overall weight and your overall height and calculates this towards the AVERAGE person. However, you may have a larger muscular mass than the average joe.
For example my BMI is 28.9 which is nearly obese.
I am 5ft 8 and 185lb yet i have 8% bodyfat (this is far far less than the average joe)