I think you also need to factor in where and how the study took place. the most recent version I read was all about NYC kids and how more were qualifying for gifted kindergarten. (not the New Yorker Magazine)

If you have a copy of the exam and a tutor and you've taken it 5 or 10 times, you will do better than an untutored child. But the same tutored child will not be as successful 10 years later when they haven't been prepped for the test.