During the "dark ages" of baseball (before 1920) fielding errors were much more prevelant than they are today. There were sometimes as many as 12 errors committed per game, quite an extraordinary number by today's standards. Due to these extremes we have normalized the pre-1920 teams to 1920 levels which will keep the number of errors under control (this makes for a faster playing game that might be preferred by some). However, for realism sake you may wish to allow these teams to commit errors at their historical rate. If that is the case then you should use this option.
See the Maximum Rules page for more information.