Baseboard meeting at top of corner but not bottom
Baseboard should be installed using cope joints, not miters, for this exact reason. Some builders request that the drywall taping crew fill the edge channel on the drywall sheets along the floor to keep things more square, but this still doesn't resolve the issue of boards tightening outward when fastened, which very often results in open miters. 

Coping, along with a slightly tensioned installation (cut 1/16" to 1/8" long, nail the ends first), almost always results in tight inside corners. This is crucial when working with natural wood, where fillers don't result in a quality appearance.