Our culture optimizes for “the average”. This isn't saying we encourage everyone to be average, but we measure from the average. This breaks down when a measure is “jagged”. Consider people of different sizes, and you're tasked to sort them from smallest to biggest. Do you do it by height? Well, this would rank a 1.5m tall obese person as “smaller” than a 1.6m very thin person. By weight? This has the same problem. As it turns out, “size” is jagged. This may seem silly, but we do the exact same thing with intelligence tests, clothing, standardized tests, and even norms. There is no “ideal” beauty measure or “smartest” intelligence measure, we measure nearly everything as if there were smooth metrics. No one is actually “average”.
The book is a fun read and isn't long. Once you wrap your mind around the idea, however, it's a bit repetitive.