If Farmer started at SS for the next three weeks his numbers would drop. So far Rocco's been spot starting him where it looks like he'll have an advantage. But starting every day means him facing more righties, more tough pitchers, fewer selected match-ups.
And just clarify the WAR thing, Correa and Farmer have .702 and .704 OPS right now (..307/.395 vs .310/.394 obp/slg) so not a huge offensive difference other than the massive number of GDP, which you'd kind of expect from a guy who has been fight plantar fasciitis most of the season.