Terrace,
I suspect you and happymom are not alone in wondering about how well cognitive assessments pick up math-related ability, as the latest revision of the WISC (the -V) adds a key fluid reasoning subtest, which is a measure of quantitative reasoning (probably most akin to algebraic thinking). In the past, cognitive assessments have been uneven in assessing mathematical thinking. The SBV has a quantitative section, which sometimes picks it up, and the DAS (and DAS-II) has a subtest in the nonverbal reasoning (similar to WISC-V fluid reasoning) cluster which addresses some quantitative thinking. But the majority of test components, most likely in an attempt to avoid measuring instruction rather than native ability, assess little math, except in an incidental way. The WISC-IV had one measure that had more direct connection to math (Arithmetic), and it was confounded with working memory. (On the WISC-V, that subtest and the Figure Weights quantitative reasoning subtest I mentioned earlier comprise the Quantitative Reasoning Index.)
IOW, the assessment field appears to be working on improving the technical qualities of cognitive assessments with regard to mathematical thinking.
And on memory: the tests you've mentioned do not have measures of long-term memory, only of short-term/working memory, which, while closely associated with mathematical ability, does not necessarily have to be exceptional in parallel with excellent encoding to and retrieval from long-term memory.
