Ain't it obvious? When you're see a different image at every screen refresh, it magically moves smoothly. At ~30 fps (or ~25fps for you pal people) the screen blinks the same frame twice, so you get a little ghosting.
As example, Sonic Heroes PC (the demo, at least) has a small bug: it picks the highest supported refresh rate for the resolution you selected (on XP, at least). So, the game was running at 60fps on my computer, but my monitor was running at 120Hz, and the ghosting effect was there. When I got my videocard to force the refresh rate to 60Hz, the game suddenly moved a lot smoother.
