[raymo]

I thought you meant that the drive motor kept going after you released the button, but Bill is spot on, backlash and inertia. All gear driven mounts will suffer this to a greater or lesser degree, unless you chance upon the one where the gears and worm have been made accurately enough for the backlash to be almost entirely adjusted out. When centring an object just change to speed 1 or 2 when you get close, and release the button a little early to allow for the over run. You could of course check and adjust the backlash on both axes, to minimise the problem. Its not difficult, just a bit fiddly; there are video tutorials on you tube. You can get a belt drive kit which virtually eliminates the
gear backlash [but not the worm backlash], and quietens the motor noise significantly.
raymo