Bedeviled by the A77 preview button
A wild guess:
You use a lens which has so much focus shift when stopped down, that it is actually visible in the preview. It will not affect the final image because the camera knows of the focus shift and automatically compensates for it.
But it is only a guess. I have read about lenses with this kind of focus shift, but I have not experienced it myself, and I don't really know how the camera will handle such a lens during DOF preview.