Advocacymom, my DD15 had a very similar profile in the WISC-IV and WIAT-II when she was tested at 8 years old. She was also tested with the WISC-IV at 6 years old, with similar results. We have run in to very few gifted kids with this profile, but over the years we were able to figure a few things out. This may or may not be relevant to your son.
1) DD is VERY literal. This has caused all sorts of issues, especially with reading comprehension. She had a photographic type memory when she was younger, and could pull out anything that she read. What she could not do, however, was relate to context. (She was grade skipped and subject accelerated, but we kept her very sheltered so she did not have the frame of references necessary to draw the correct conclusions.)
2) DD was a very early reader, and could phonetically attack any word and her word per minute count was off the charts. However, the schools she went to did NOT teach vocabulary, so there were many words that she knew how to read, but did not know what they meant.
3) On the WIAT, she basically had top scores on the Math and Writing broad scores. Everything was high except her Reading Comprehension, which showed up as a relative weakness.
4) Like your son, my DD had a high processing speed. As a result, her reaction speed would often result in "mis-clicks" on computer tests or she would make careless mistakes because she did not read thoroughly. She still has the "need for speed".
5) With gifted kids, everything comes easy in the early years, but as educational material gets harder, they may not fully understand everything. (Teachers typically blow GT kids off, and concentrate on the lower learners, and I can tell you how many times my DD has been told to "figure it out by herself".) The GT kids don't want to "look dumb" by asking questions, so they interpret things their own way (right or wrong), which will often result in a knowledge gap later in life.
If I were you, I would request that the teachers answer his questions and try to help him overcome any of his relative weaknesses. Try to increase his vocabulary (online vocabulary builders work great). Also, have the teachers go back and revisit any comprehension questions that he misses. This is a great tool for learning HOW his mind works and brain thinks. Also, I would suggest that they keep "feeding" him math topics at higher levels, and possibly look at subject accelerations or full grade skips, while keeping in mind, they may have to go back and fill a few knowledge gaps here and there.
It has definitely been an interesting journey for us! Good luck!
