The notion that abilities de-correlate at high range is known as Spearman's law of diminishing returns . A simple analogy from sports: ability to run marathon positively correlates with the ability to sprint, in the general public, but among the world class athletes, you run into genetic variations which trade sprinting performance for marathon performance and vice versa.

Another point is that IQ tests have to be neutral with regards to the background skills or knowledge, which has a very unfortunate side effect of not measuring performance of the mechanisms involved in forming or applying skills and knowledge. By the way, on a Gaussian prior, poor correlation implies very substantial regression towards the mean.

edit: cut-n-paste error in URL.