The force applied to the boss face creates a deformation that is not uniform across the face of the boss. There is some variation in the deformation result on the boss face. The graph is only plotting the maximum deformation in the structure. Where is that maximum located?
A displacement applied to the boss face enforces a perfectly uniform deformation. It requires more force to keep the whole boss face at the same displacement value that was obtained as the maximum singe point in the applied force case.