When an individual at first glance is of a normal height/weight ratio but then on further inspection is actually a fat ass.
With men, this happens often with an ample chest cavity which hides a beer gut. This theory can also be applied to women.
The most difficult cases arise when the subject is very hot in the face therefore making their body less prominent but then only with another glance does their actual shape come to light.
"That guy I brought home last night looked so hot when he was dancing on the bar, but that well toned chest was hiding a whole world of secret-fat."