A simpler way to look at SSAT is that the percentile among same-grade test takers is accurate no matter what (the test makers make sure of this result), whereas the test-taking population varies per grade, especially for the 8th grade (about 5 times greater population than in other grades).
That means, if we assume 5% of entire 8th graders take the SSAT, only 1% of entire 6th graders take it.
Now assume only “top portion” of each grade takes it (“top portion” meaning not IQ or talent but sheer “test-taking skill” which I think is a reasonable assumption among 6th, 7th, 8th graders - for whom SSAT is considered the most used and thus important standardized yardstick. (ISEE is another, distant second.)
That means, if we assume top 5% of entire 8th graders take the SSAT, only top 1% of entire 6th graders take it.
Being top 1% (ie 99th percentile on score report) among top 1% of 6th graders would be (ie 0.01%) theoretically 5 times tougher than being top 1% among top 5% of 8th graders (ie 0.05%).
The difficulty of obtaining a high percentile score entirely depends on how other test takers in your grade do, and how others do is, by our assumption, the dictate of the size of the test-taking population in your grade, at least among 6th, 7th and 8th graders - the grades in which SSAT reigns supreme authority.