That chart only aims towards people 5'4".
Over these last few decades, people have been getting taller, averaging 5'10".
Of course, its no excuse, but shorter people tend to be more obese, and there are less of them.
But besides that, its going by BMI, which I've explained in the BMI topic that it doesn't care about density of the body, it only cares about mass all together, so people with a large amount of muscle are considered obese.