What’s up with all the perfect games lately?

Philip Humber of the Chicago White Sox pitched a perfect game against the Seattle Mariners yesterday. He faced only 27 batters, and got them all out. It's an exceedingly rare feat—Humber's was only the 19th in modern Major League Baseball history—but not as rare as it used to be. Or is it? (Click on the chart to view a full-sized version.) In the first 60 years after the turn of the 20th Century, only four major-leaguers  managed to pitch perfect games; 15 have done it in the 62 years since. It sure looks as if pitching a perfect game got easier around 1980,...