It is sort of meant to be a mystery, as bWAR and fWAR are based on proprietary formulae, so try as we might, there is only so much that we will understand. But some areas that probably fuel the discrepancy are the positions (Baty has played second and third while Vientos has played third and DH), the related defensive workload (Baty has played 776 innings and Vientos has played 491), double plays (Baty has grounded into six and Veintos nine), basestealing (Baty has stolen six bases without being caught, and Vientos has yet to attempt), baserunning beyond stolen bases, etc. I might guess the on-base percentages are a big factor. A .294 figure versus .309 may only a be a 15-point difference, but that can be meaningfully significant if one number is on the positive side of the replacement-player line and the other is not.