Because each time the obesity crisis is discussed on TV, they show you images of 400 lbs landwhales. So they watch this and think "hey I'm just 220 lbs, so I'm just fat". Also the term "overweight" was not a great choice, since "over normal weight" would seem to cover everything from BMI 29 to 99.  So they correctly believe themselves to be over the normal weight, but when told that they are actually obese they are surprised because they can still walk unassisted. It probably makes sense when telling them their BMI, to first give them an idea what the weight ranges could mean. Something like: overweight is unhealthy, obese is severely unhealthy and morbidly obese is life threatening. Or: once you are in the obese range you have increased risk for high blood pressure, type 2 diabetes, coronary heart disease, stroke, gallbladder disease, osteoarthritis, sleep apnea, breathing problems, cancer, depression and body pain.