Many IQ tests are suitable for a brain prepared to certain things: some
abstraction, fast responses under time pressure, asociation, sequencing,
simple calculations or evaluations, stepping, finding little details...
Maybe Chess practice is a way for conditioning brain to these kind of
tests, and this is the reason of success. Brain can be educated and
trained for many things. It is possible that Chess and other board games
act as teachers for the brain in an indirect way, making possible best
performances in tests in which many of the common mental proccesses you
apply in a game are tested.