relax and forget about the BMI, because it does nothing to take into accont your fitness level or body fat percentage it really isnt of much diagnostic use. they even say so on their own page. they give the example of the 6'3" 230 lbs body builder and the couch potato with the same stats. they bothh have the same BMI.
if you are really worried about whether or not you are lean enough, go get your percent body fat measured. the simple caliper method or the ultra-sound method can't be that pricey....you might even find a trainer at a local university, HS or gym that could do the measurment for you really cheap.