With all due respect to Don Larsen, who pitched the only perfect game in World Series history, the one that Chicago White Sox pitcher Mark Buehrle threw Thursday was even tougher.

In fact, it was the second-most difficult perfect game of all time—and not merely because of center fielder Dewayne Wise’s home-run robbing catch in the ninth inning.

In throwing his perfect game—the 18th in major-league history—Mr. Buehrle stifled the Tampa Bay Rays, whose team on-base percentage (.350) is the second-highest of any perfect-game victim. On-base percentage measures how often players reach base via a hit, a walk or getting hit by a pitch—and thus the ability of the opposing offense to prevent a perfect game. Although there are other events that can spoil it—an error, for example—those are more under the defense’s control.

By this standard, the most difficult perfect game ever was thrown by another White Sox pitcher: Charlie Robertson, against Ty Cobb’s Detroit Tigers in 1922. Mr. Larsen threw his historic game for the New York Yankees in Game 5 of the 1956 World Series against the Brooklyn Dodgers. But Brooklyn’s .342 season ending OBP ranks fourth among the perfect-game victims.