The gap between performance and price is the whole product
Performance explains most of where a player is indexed. The interesting players are the ones sitting a long way off the fitted line.
Fit a line through performance against index across the whole universe and it explains most of the variation. That is reassuring and slightly boring: it means the market is broadly rational, and that a model reproducing it is not discovering anything.
What the line does not explain is where the work is. The residual — the vertical distance from a player to the line — is the part of his price that his output does not account for.
Reading a residual honestly
A player well above the line is not automatically overpriced. He may be young, on a long contract, playing in a league the model discounts, or doing something the metrics do not capture. A player well below it is not automatically a bargain, for the mirror-image reasons.
The residual is a question, not an answer. It tells you which twenty names out of several hundred are worth an afternoon, which is exactly what a screening tool should do and considerably less than most of them claim to do.
Why the scatter is drawn, not summarised
A correlation stated as a single figure hides its own shape. Drawn as a scatter, you can see whether the relationship holds across the range or breaks down at the top, whether the misses are symmetric, and how much of the cloud sits far enough from the line to be worth looking at. All of that is invisible in a number.
The line is the consensus. The distance from it is the disagreement, and the disagreement is what you are here for.