emigee, I've had the same thought about Triple Nine. Mensa has relatively low standards, so that one doesn't surprise me as much. The rationale, I believe, is two-fold: 1, the population that took these tests in those eras was pre-selected for the upper few percent, so the tests effectively spread out the right-hand tail enough to discriminate scores above +2 SD; and 2, the earlier tests were more aimed at "aptitude", meaning reasoning ability or learning potential, which better approximates the same qualities assessed by IQ tests.
The first assumption has changed in recent decades, since the push for college for all (SATs and ACTs), which has since morphed into post-graduate for all (GREs). Some states even use the SATs as a state-mandated high school exit exam (much to the delight of the College Board), which means that every single high school student is expected to take them, a far cry from the data pool up through the early 80s, which was a relatively stable top 10-15% of the educational population (sadly, with no allowances made, of course, for economically, socially, or instructionally disadvantaged populations with no access to the test or its preparations).
The second assumption has changed as the test has become more focused on acquired skills, partly, I suspect, in response to the movement away from liberal arts, and toward school-to-work in education.
