However, removing unearned runs doesn't adequately adjust for the effects of defensive support, because it makes no adjustment for other important aspects of fielding, such as proficiency at turning double plays, throwing out base stealers, and fielding range.
The rules give scorers considerable discretion regarding the plays that can be called as errors.
[6] While ERA doesn't charge the pitcher for the runs that result from errors, it may tend to over correct for the influence of fielding.
During the early days of baseball history, this over correction for fielding errors caused pitchers on bad teams to be overrated in terms of ERA.
For example, Ron Guidry—a flyball power pitcher—and Tommy John—a groundball control pitcher—were teammates on the Yankees from 1979 to 1982, supported by the same defense.