Consider the sample source.
Millionaire mind studied successful multi-millionaires right now. Majority of whom are middle-age. Ie they went to College quite a long time ago. I strongly suspect that competition for college admission is much greater today than it was then. I based this simply on that fact that the population is simply so much larger today and hence almost automatically increased competition. Thus I suspect that in 10 maybe 20 years when a new version of the book comes out the scores will be significantly higher. Makes sense to me.