I don't think the scores vary that much when you consider the extreme ends of the spectrum. In the book, the concern was that kids who are not really gifed (over the long term) are identified in preschool and kindergarten. So then what happens in third or fourth grade when their scores drop? The school sends a letter home saying "your child is no longer gifted?" Plus, kids who end up being gifted later may never be identified, or the "slots" in a program are already filled up. It's a dilemma.
I read the book a long time ago and don't have a copy, so don't remember specifics, but here's a short summary
http://nurtureshockmoment.blogspot.com/
