Exactly. Skinnyfats can sneak under the BMI really easily. All it takes is a sedentary lifestyle. Anyone in school or working a desk job is going to have an abnormally low amount of muscle unless they're putting effort into a consistent exercise routine. And once muscle is removed one can have a shocking amount of fat while still sitting inside the "healthy" BMI. It's ridiculously easy for sedentary fat people to come in under the upper limit there.