I'm on my phone vs a computer so I'll be brief, but I don't think that having gifted+ kids statistically over represented necessarily means that they will all be squeezed into a normal distribution thus falsely depressing the scores of all but the few highest scorers. I'd need to look into it further, and will try to later on a computer if someone else doesn't beat me to it, but I'm not sure that the goal with the norming group is to have a bell curve where there is only 2% of the group scoring in the top 2% of the test's ability level otherwise they really should have just taken a random sample of kids not intentionally recruited outliers.
