One of the differences between the WISC and the SCAT is how it's administered - the WISC is typically administered by a psychologist 1-on-1, so that a child is going to have a bit of a touchstone for focusing. The SCAT, if it's administered everywhere as it is here, is taken on a computer in a testing center, and the child has no prompting or encouragement or whatever from anyone to pay attention, or to not fly through or to not stop at some point if they simply feel like stopping or not answering a question. At 8 years old, most kids haven't had any coaching in how to take tests like this either, so he might not have thought to go back and check his work or to leave a problem he didn't understand until last and then go back and try it again, things like that. I'd ask him if he remembers if he ran out of time before he answered all the questions.
I *don't* know how ability as measured by IQ correlates with SCAT scores - IQ tests are typically normed by age, and the SCAT is measured against above-grade-level students. MY EG ds had SCAT scores that are in line with his WISC/etc, but my younger dd who isn't quite as EG basically bombed the SCAT. OTOH, she took the test when she was in 2nd grade, and as time has gone by, she's shown to be very scattered with test performance - which in her case may be related to a reading challenge.
Another thing I wonder about - were your ds' WISC subtest scores even across the board, or was there a lot of variation? If there was significant variation in his scores, it's possible an area of relative weakness might have impacted the SCAT score.
I would have him retake the SCAT whether or not you care about signing him up for CTY courses if you don't mind paying for it again, and talk to him first about being careful to pay attention to detail or be sure to answer all the questions etc - things like that, *if* you think the difference in scores might just be due to inexperience with standardized testing. It's a relatively inexpensive test, so it can be a good opportunity to practice test taking skills.
Best wishes,
polarbear
