Why does iPod sometime not unmount after sync?

I have noticed that sometimes my iPod doesn't unmount from the Finder after doing a sync. It is NOT set to FireWire disk use, and when I try to eject it from the Finder, I get the message that the iPod is in use by another application. Even quitting iTunes doesn't resolve it. Sometimes, if I right-click in iTunes and select "Update iPod", it will unmount.

Can anyone tell me why this happens?