I'm no expert, but I would guess that there is a bit of optical illusion present in all pitches. We always hear about how a pitch needs to be deceptive or the hitter will be all over it. The key to a pitchers success is making every pitch look the same to the hitter even though the various pitches are very different. I think much more goes into that deception then just this curveball effect they talk about, but it seems to make some sense.