@1111 WPPSI can be accurate but it can also be way off. One of mine kid's WPPSI was very close to his WISC all the way through childhood. The other had a invalid test, by the notes from the examiner, he was not cooperative during the test. If you had scored it it was lower by 2+ SD off of his WISC results. Younger kids can be all over the place on WPPSI. Their cooperation and comfort level can impact the results a lot. If they are perfectionist or shy the results can be lower. A lot of factors involved with a younger kid.
I don't know about overestimates, but I've seen underestimates firsthand. A good examiner will make notes in the report about issues that came up during the test.
