If your pitchers are from bad-fielding eras, especially deadballers, your SS will not field as well as if they are modern-era pitchers. If you're in a high-cap league, hitters will put more balls in play and create more fielding chances, leading to more errors than the SS had in real life. And if a few teams in your division/league went heavy on speedy contact hitters, that will also increase the plays at SS. At the very least, if you have a few pitchers doing poorly, that leads to more chances.