I saw a psychologist speak at a GATE conference recently who said that PSI in gifted kids is simply a measure of how much they are affected by perfectionism. Now obviously, that is a bit of a generalization but it does demonstrate how a child with a high PSI may be able to go through those sections quickly and with accuracy, without being overly worried about correct answers or how the tester is viewing them, if they are the fastest or slowest ever or what someone will think if the answer is wrong... or the multitude of other things my DS says were racing through his head at the time!