Depth of field (DOF) is proportional to the f/no. of the optical system, so an f/1.4 lens has less DOF than an f/2.8 lens, all other things (focal length, distance to subject) being equal. My understanding is that even though you have an f/1.4 lens mounted, the viewfinder optical system has a larger f/no so the viewfinder optical system has larger DOF than the lens/sensor optical system.
Thanks for bringing up this oddity. I will have to try it out myself as I have never noticed it.