Every application has some degree of stuttering. It's usually caused by stalled updates for one frame due to some other application using up GPU or CPU resources.
It's also why in fullscreen mode you usually get less stuttering because there you more or less have exclusive GPU time for that monitor.
If you use a fixed timestep the movement of the object isn't dependent on the frame rendering any more, but it just keeps going and whenever there are resources available it will be drawn to the screen.