I don't know about you, but I move around a _lot_ during sleep. Sure, if it only knows that I'm awake if I go to the bathroom or kitchen during the night, I understand the difference. Would be interesting to know how it makes the distinction, though.
You (supposedly) only move around when you're in certain sleep stages, and not others. When you're moving around, you're still asleep, but you're much more lightly asleep than when you're not moving around, so it's a much more optimal time to wake up and you can also track how "restful" your sleep is.
Typically, when you enter deep stage sleep your brain turns your normal motor functions off - it's supposed that this is to stop you from hurting yourself while in deep sleep (although some people suffer from sleep disorders where they do move).
In lighter stages of sleep this doesn't happen and you do move around.
