Underweight = Less than or equal to 18.4
These males are the same height and weight, so therefore, they have the same BMI and they are both classified as obese. But clearly, the male on the left is not "obese". He is in great physical shape and has a great amount of muscle mass. It is important to remember that muscle weighs more than fat.
So when figuring out your BMI, don't get too caught up in the numbers. BMI is just a good way for most people to classify their weight to determine if it is healthy or not. It is also good to keep in mind that if someone is classified as "underweight", they can be more at risk for disease/complications than someone who's weight is classified "normal".