A child may have DYS level IQ, but if he/she doesn't have a support system that helps 'utilize' his / her potential, the achievement test may not be as high as what it could have been. So, I would guess that DYS level achievement test result is more rare than DYS level IQ test.
This was the case with ds. He has been tested with WISC twice, and both times he easily hit the DYS qualifying scores. The first time he was almost 6 had about two months of kindergarten under his belt. The second time he had one year of school behind him.
He's home educated, and I was in denial about his giftedness (and his gifted needs) all along. I never pushed him at all, and even held him back a bit before grade 1. When he had the second WISC, he also took the WJ-III. His broad score was one point below the DYS cutoff. His math really hurt him, as he has not been exposed to much at that point. He was above grade level, but not enough for DYS. In his case, the lack of exposure suppressed his achievement scores.