Scores differ between the tests (Elementary, Intermediate, Advanced), but 6th/7th/8th all take the same Advanced test, so scoring scales are the same. It's similar to 10th graders taking the PSAT and 11th graders taking the SAT not having directly comparable scores, but 11th graders and 12th graders (and 7th graders) taking the SAT all being on the same scoring.
Reported ercentiles will differ, as they are compared to different above-grade cohorts, but the raw/scaled scores are the same.
Since 5th and 6th graders take different tests, those scores aren't directly comparable. So 499 in 5th and 499 in 6th aren't the same, but 499 in 6th, 7th, 8th are the same (assuming the test version was per JH recommendations)
explained here:
http://cty.jhu.edu/talent/testing/about/scat.html - Scoring and Timing
I've emailed JH to get an answer to my original question.
