2. Calculating Defense: Each system uses a different defensive metric. fWAR uses UZR, while rWAR uses TotalZone (developed by Sean Smith). UZR is more accurate but is only available from 2002 onward, while TotalZone values can be calculated for any player in baseball history.
As a result of these differences, rWAR values typically come in lower than fWAR values, meaning a 6 rWAR is more impressive than 6 fWAR. For more on the differences between fWAR and rWAR, please read these links: The Book Blog discussion, On Fielding Differences, and On Scaling.