In Section 3.6, I said that T scores are designed to have a mean of 50 and a standard deviation of 10 and that the Achenbach Youth Self-Report measure produces T scores. The data in do not have a mean...

In Section 3.6, I said that T scores are designed to have a mean of 50 and a standard deviation of 10 and that the Achenbach Youth Self-Report measure produces T scores. The data in do not have a mean and standard deviation of exactly 50 and 10. Why do you suppose that this is so?