One thought, here-- do you have any indication that she has high potential in mathematics, beyond the achievement scores taken at the end of each school year? Those SCAT scores seem to pretty accurately mirror the MAP scores from this spring, so unless you have additional data, I'd have to think that they are accurate.
It sounds like she's been fortunate enough to live in a significantly enriched early childhood environment, yes? So relative to peers who entered school with very poor numeracy skills, that 90th percentile score at the end of 1st makes quite a bit of sense.
It's entirely plausible to me that as she has had less and less instruction ahead of the instruction being offered in-level (at school) and that this explains why her scores are drifting toward the mean in her age cohort.
It's possible that this could a painfully awful fit between her learning style and that offered by the curriculum/instrction. She's outstripping the knowledge that she gained BEFORE school took over math instruction, perhaps. What math curriculum does her school use? What did she see prior to K/1st?
I guess I'd try looking at how INTERESTED she is in learning/doing mathematics to know what to do (if anything).
It's entirely possible to have fairly normative development in one cognitive domain and very asynchronously developing ability in another, too. If that is the case, then I'd probably just accept that subject acceleration in language arts would be a good path forward for her-- it would meet her needs for advanced instruction in the area where she needs that. I'm not sure that those math scores indicate a problem to be remediated. In my opinion, forcing the issue isn't likely to help much, and may even make things worse by turning it into a "flaw" in her mind.
ALSO possible that this is the emergence of a learning challenge of some kind as students move from concrete to increasingly abstract (representational/symbolic) thinking in mathematics. It might not even be a learning challenge so much as that not all people have the same affinity for that symbolic quantitative thinking.
Hard to say.
I'm guessing that your gut is telling you something is off, though, or you wouldn't have asked.
