The biggest difference between your state testing results and the SAT/ACT for this age group is that state testing includes open-response questions, including expository writing. SAT/ACT do not. What is described as "writing" is still multiple choice, and requires no actual writing (unless you do the optional essay, which typically is not expected for talent search testing, such as TIP). I would expect that, when you look at your DC's score breakdown for state testing, the multiple choice sections likely are stronger than the OR or expository sections.
For example, on the grade four test in your state (beginning with last year's test), 25% of the score is based on revision (multiple choice), and 50% on editing (multiple choice), while the remaining 25% is based on a single writing sample (composition). A student could meet criteria without doing well on the writing sample, but they would have to get every single mc item correct. On the grade 7 test, 35% of the score is based on the writing sample (with the composition raw score doubled in the calculation of the scaled score), and it is impossible to meet criteria without obtaining at least a few points on the composition.
I would guess that your child's skills for generating accurate writing samples are not at the same level as for identifying errors in mechanics, grammar, and syntax in other people's brief writing samples.
