Last week I presented an overview of the concept of "regression to the mean" as presented in Gary Smith's new book, What the Luck: The Surprising Role of Chance in Our Everyday Lives.
The basic idea is that outlier performances — whether good or bad, high or low — tend to be followed by performances that are more average or more representative of one's actual ability.
The reason is that exceptional performances are usually the result of a big dose of either good or bad luck, which will not continue, due to the fluctuating nature of randomness.