BMI isn't the most accurate way to measure one's overweight-ness. I'm 5'4 and 150 lbs (plus tax!), and I think that puts me over the obese portion for my age, yet I have only ~9% body fat (9% according to water dislacement, 11% according to caliper).

BMI assumes that a large chunk of the weight is fat (hence its ratio of fat to height), but if a lot of one's weight is something else (ie muscle) it throws the whole thing off.