Teams with hard-hitting, opportunistic defenses and ball-control offenses can use turnover differential to keep games close.
Luck also plays a role in turnovers, and sometimes that luck can run out.
Looking at the ESPN Stats & Information chart, which shows turnover differential through Week 4, the Baltimore Ravens have to feel pretty good about where they stand. Winning or evening out the turnover battle could make the Ravens enough tougher to beat (Baltimore has forced only two turnovers so far).
The San Francisco 49ers and Arizona Cardinals are minus-6 in turnover differential. Both have played three of their first four games on the road. Both have suffered from shaky play in the passing game. I would expect their turnover differential to improve as they play more games at home.