Modern pitchers = less errors
The year of the batter also comes into play, with modern again yielding a lower chance of error
From the knowledge base:
We also use Log5 normalization for fielding and determining errors. The 4 key pieces here are: fielder's fielding percentage (FPCT), the league average from the fielder's season at the position, the league average from the hitter's season at the position and the league average from the pitcher's season at the position.