The WJ, like the WIAT and the KTEA, which are the other two most-commonly administered norm-referenced measures of achievement, is intended to sample skills in order to make a comparison against the norm group (presumed to be representative of our population by age). It is not comprehensive, nor is it necessarily aligned with any given school system's curriculum.
With reading, the foundational skills have usually been acquired by the third grade level. Beyond that, it's essentially refinement of the same skill set, through acquisition of vocabulary, increasing fluency, and applying overall cognition to comprehension. There isn't really any specific content in ELA, up until high school, when an expectation of a common literary experience begins to come into play. This is why there is often a better match between reading scores on the WJ, et al, and on curriculum-based measures.
Mathematics is a bit different. After mastery of basic arithmetic, there are numerous qualitatively different skills, for which sequence matters. Many of them also involve specialized math vocabulary that most children won't have exposure to in their non-instructional environment. How this often plays out for a gifted youngster is that the math reasoning (e.g., applied problems on the WJ, math problem solving on the WIAT)score is quite a bit stronger than the calculation score, because one can think one's way through real-life math situations without knowing the formal notation or vocabulary of math, but it is more challenging to compute problems when you don't know what the symbols mean because no one has told you the conventions.
In addition, our educational system's expectations for elementary math are pitifully low, so a child who has mastered through long-division and multi-digit multiplication by first or second grade age (not that uncommon an occurrence among kids gifted in math) will blow any computation test out of the water, because their age-peers are still working on addition and subtraction without regrouping, and the skills they have mastered are not part of the curriculum until fourth and fifth grade. These high scores do not actually mean that the child is ready for the grade-equivalents reported, both because of the spurious nature of grade-equivalents (but that's another story, for another thread), and because there are other math skills in the curriculum, that are not being sampled by the test.
For a more comprehensive math assessment, I would suggest the KeyMath-3DA, which can actually give you some instructionally-relevant data about specific skills, while retaining the psychometric robustness of being a nationally norm-referenced standardized test. I've mentioned elsewhere that the best way to decide on grade/subject acceleration in a particular setting is to use curriculum-based assessment, meaning formative (progress monitoring) and summative (standards or outcome) assessments drawn from the actual curriculum being used in the proposed placement. It's even better if there are local norms for the assessment. This helps to correct for the communities where everyone is above average.
