Wikipedia is Wikipedia, but let's see here:

"The body fat percentage is a measure of fitness level, since it is the only body measurement which directly calculates a person's relative body composition without regard to height or weight. The widely used body mass index (BMI) provides a measure that allows the comparison of the adiposity of individuals of different heights and weights. While BMI largely increases as adiposity increases, due to differences in body composition, other indicators of body fat give more accurate results; for example, individuals with greater muscle mass or larger bones will have higher BMIs. As such, BMI is a useful indicator of overall fitness for a large group of people, but a poor tool for determining the health of an individual."

More or less exactly what I've been saying. As for numbers, my current BMI is around 32, with a fat percentage around 24%, which is clearly overweight, which I've never claimed not to be. With an ideal fat percentage of around 15%, my BMI would be around 28,5. Make of that what you will. As for when it comes to muscle mass, you seem to have trouble understanding what the word "naturally" means. Sure if all I did was lay on a couch I'd probably have significantly less of it, but a heavy frame takes some power to move around, and I've always been on the move a lot. You have muscles all over your body, not just in your arms you know.