This looks like the variables are reversed. If V is the intended approximation of U, heavy selection is on high values of V, not U. That selection tends to hit places where V diverges upward from U, so V is an unusually poor approximation of U — not the other way around.
This looks like the variables are reversed. If V is the intended approximation of U, heavy selection is on high values of V, not U. That selection tends to hit places where V diverges upward from U, so V is an unusually poor approximation of U — not the other way around.