I agree that the movement "seems" to be NOT totally synchronized when render same image is checked. To me this is becomes more apparent if you hold down the F6 key... you get a kind of a fast motion SS... The second monitor fan has a very rhythmic repeated motion (the rear fan especially) On the main monitor this fan is not as rhythmic.
I think
After a few hours.. thinking
.. I am speculating that with "render same image on both screens" checked we are actually seeing a second copy.. with a few clock cycles apart.. maybe