I think it's probably because OOTP uses a WAR generator more similar to fWAR than bWAR. His peripherals are strong so he has the second-best FIP in the league but with considerably more innings than the FIP leader.
I've never really understood tying WAR to FIP for pitchers since the bottom line in value should be output in the end. Yes, peripherals give a better understanding of how well someone pitched, but the actual value they provided is tied to the runs they did give up, not the runs they should've given up.