Good point.
My own observations lead me to suspect that it is the rate of acquisition that probably points to the underlying ability of the learner. Otherwise, it's probably a matter of exposure alone.
So it isn't reading level that matters really, so much as how rapidly they PROGRESS. Well, those differences more or less vanish entirely for a PG child by about age four to six, since they are reading at an adult level. How does one "progress" at that point in terms of reading level? The options are limited.
With mathematics, there is a much clearer way of measuring progress for much longer, since advanced mathematics continues quite a steady learning curve through early adulthood.
It's hard to point to "adult level" mathematics, if that makes sense. Is that basic algebra? Calculus? It depends, but even most PG children don't get there until they are 4-10 years old.
What is "adult level" reading? A newspaper? A best-selling novel? Many PG children are already there by the time they are chronologically kindergarteners.
I guess I'm framing this as an assessment ceiling issue as much as anything else. An assessment with a low ceiling (like literacy, in this context) is not going to be very useful as a differentiator. Mathematics has a much higher natural ceiling, so it is probably a better tool for correlation to IQ.
It also seems to be that literacy acquisition is something which is difficult to truly assess well, which might be why it doesn't correlate with IQ as cleanly as mathematics does. It's harder to break out the individual skills and differentiate them with objective criteria, which leads to a lot of noise in the data when children are assessed.
A child can "read" (meaning decode and pronounce) text that s/he can't really "process" yet, too. That really isn't possible in mathematics, so inflation in scores isn't possible from that kind of artifact.
