Not an expert, but I imagine that you wouldn't want an achievement and IQ test too off the mark. This seems more likely happen if a child has a learning disability or a more global issue like ADHD.
My DS8 had a WISC 141 FSIQ, 146 GAI, and a 154 achievement on the WJ. *However* his math score brought the entire WJ up by a lot. His reading and writing scores were in the 135-145 range. It was surprising because his verbal skills on the IQ test far outstripped his perceptual reasoning score. I think it shows that the kids are still developing-- I recall the administering psych (WISC) stating the the perceptual reasoning score was probably low and would increase with age.
That's just our story- when I look at your DS's achievement test, it seems pretty clustered with the initial and subsequent IQ test. Keep in mind too that some scores may improve with experience- spelling seems like a good example of a score that will go up. Math may also go up-- my DS had an early passion for math that we fed, so in retrospect, it makes sense that the math score could be high right now and plateau a bit later.
