I believe that you need to have both IQ >= 145 and achievement globally at an equivalently high level (which may or may not be +2-3 years, and being ahead in one area isn't sufficient), not one or the other. My personal guess is that there are a large number of kids who have one but not the other, for one reason or another.

I'm curious how you concluded that +3 was the achievement cutoff. The DYS cuts for 7th graders are well above the 50th percentile for 11-12th graders, which would be +4-5.