Agree with above and will add to this that sometimes kids can’t finish all the problems in the math section. Also, if your kid is advanced he/she may have not had some of the test items in a year or so. There is variability in performance on the SSAT between tests.
Often it’s a timing issue. The first time our kiddo took the test it was at a school far from our hometown - no clock in the room and the kids were not allowed to wear their watches. There was no way to time oneself or keep pace. Very unfair, as a testing center should have a clock.