The psychologist who tested my son said that it is easier to score very high on tests when the child is younger, rather than older. They only need a few more right answers to score very high.
For example, some of the tests may test reading or pre-reading. If the child is an early reader, they will score very high on that part. However, other children later on may catch up as they also read, etc.
The discussion reminds me of my friend who was so proud that her son walked early. He walked at 9 months. All 3 of her kids walked at 9 months. Great! However, now that the kids are older, although they are doing well, etc., it's not like the kids have gone onto the Olympics!