There are different levels of the test, and it appears percentages are comparisons to results of students in the higher grades the test was designed to measure. There's this from the CTY website:
CTY uses three levels of the SCAT:
Students in grades 2-3 take the Elementary SCAT designed for students in grades 4-5.
Students in grades 4-5 take the Intermediate SCAT designed for students in grades 6-8.
Students in grades 6 and above take the Advanced SCAT designed for students in grades 9-12.
Because this is an above-grade-level test, after the test, you’ll receive information that shows how your child’s score compares to that of students in the higher grades for whom the test questions were originally designed.
