Just some perspective:
1. The gain in Quant is only .3 of a SD (5 standard score points), so not statistically significant. Nonverbal, on the other hand, has gone up by 1.2 SD (18 standard score points or more). So it's just the Nonverbal section that's gone up.
2. The CogAT is not a true IQ test. It explicitly measures learned abilities, so, yes, it can be affected by achievement. Also, I've often seen noticeable differences when moving from form to form of the CogAT, despite the claims that the USS is a continuous measure across age/grade levels.
