Regression to the mean Prediction involves error. Height and weight are not perfectly correlated. The correlation coefficient is about .5. When we predict one from the other, we cannot predict perfectly.
Most people are in the middle of each range.
Few are at the extremes. When we pick those who are extremely tall or extremely short and try to predict their weight, they will not necessarily be the heaviest or the lightest. The errors will go toward the middle, because that is where most people are. This is regression to the mean. This is shown in the following two graphs.