It might be that your son is more of a high achiever type than you think; high achievement at a young age doesn't prove a child is gifted, just tends to support that hypothesis. Still, a 129 full scale IQ is really still on the low end of gifted, not just bright. In addition, the extremely high working memory could explain how he is able to learn quickly, and would probably give a boost to just about any cognitive task. In other words the scores might be pretty accurate and still point to someone with very unusual thinking ability, but still not perfectly agree with your preconceived notions about your son.
It might also be that he just tested poorly for some reason-- bellyache, bad rapport with the tester, unfamiliar experience leading to anxiety, learning disabilities, etc. The chance of a generally bad test day might be less due to high scores on some of the subtests-- if he had poor rapport with the tester or high anxiety, for example, maybe his working memory wouldn't have tested so high. Do you have other test results that would tend to show that these are inaccurate?
