Because the metric used to categorize is too simpleton (BMI) and unfairly counts more muscular frames into same categories. Muscle weighs more than fat.

When this guy goes to get a physical, he will go into database as "overweight" (BMI 26-27): http://ow.ly/KnTx30bsc3d

And this is "Obese": http://ow.ly/33ls30bsclV