In the developed world, people's perceptions have changed. Most people are 30-40 lbs overweight. So if you put someone who is 30-40 lbs overweight that crowd, people wouldn't consider them overweight. They'd consider that average.
The goalpost has moved. A person with a 23-24 BMI, for example, would've been considered average a few decades ago, but now many people consider that "skinny". And someone who is 21 BMI, while still in a healthy weight range, would be called anorexic. People are generally not seen as overweight unless they're morbidly obese.
The % of Americans who consider themselves overweight is the same now as 20 years ago. Yet the % of Americans who are overweight or obese has gone up significantly. This means people consider their weight relative to the population rather than in absolute. Only 25% of Americans have a normal BMI. 3 in 4 Americans are overweight.
Some will argue that BMI overestimates how fat people are because it doesn't take muscle into account. This is true for a small portion of the population that actively bodybuild. But for most people, it may overestimate it. If you're a 180 lb athlete at age 25, but you become sedentary. When you're 35 or 45, you may still be the same 180 lbs, but with a higher bodyfat percentage.