Standard deviation ... has fallen steadily year on year and decade on decade

You can see a very clear example of this in baseball. Scoring levels have stayed relatively constant in baseball for 100 years. A good batting average in the 1930s is still a good batting average. But if you look at the standard deviation (a measure of variance, of how spread out the individual scores are) of players’ batting averages by season, it has fallen steadily year on year and decade on decade. Players have collectively averaged the same, but the averages of the best and the averages of the worst each year have got closer and closer together.

N. Leamon & B. Jones, Hitting Against the Spin: How Cricket Really Works (2021), loc.2,338