I can't speak for the CCAT but it sounds like it's similar to the CogAT. On the CogAT a kids standard score can go way down just by missing a few questions. So if he was simply unlucky and got a few vocabularly words (or whatever) that he doesn't know, that could have brought down his score. To get a 99.9 percentile on the CogAT it has to pretty much be perfect. There is a low ceiling.