Cards are usually swiped in a somewhat smooth motion. Even then, it's easy to fail the motion, and you often need to swipe again.

The anti-skimmer moves the card very fast in short bursts, so while it may seem just a bit jittery (averaged out), the skimmer will end skipping whole pieces of band or losing sync.

Maybe they could use small rubber rollers to detect velocity and sync appropriately, like in a mouse.