The psych put in the report that he was making sure every line was perfect, and talking through things out loud, so she thinks its a perfectionist issue.
Talking things through out loud sounds more like an NVLD compensatory strategy to me, which would be compatible with something in the DCD/dyspraxic category of things. He has strong verbal reasoning, and may have figured out that he can use it to support areas of weakness, such as, perhaps, visual motor integration/coordination.
On math facts/automaticity: some of the same cognitive processes are involved in terms of attaining automaticity with math facts and with fine motor coordination tasks like handwriting. His working memory is good, but that doesn't necessarily mean that skills are being readily encoded into the aspects of long-term retrieval that have to with automatic tasks (as opposed to interesting, meaningful, informational content). You can't hold things indefinitely in working memory.
On re-testing: You could have him re-tested sooner, if your examiner has the WISC-V on order, as I'm told by the publisher that it should be shipping in October. Can't do the WJIII again immediately, but, then the WJIV is also out this fall. And there's always the WIAT-III or KTEA-II or -III. So if you need a short-interval re-test, this is actually a good year for it.
