Understanding How iTunes Updates Playlists on the iPod

When iTunes updates a playlist on your iPod, it takes a “snapshot” of that playlist and places it on the iPod. If you change the playlist in some way, the next time you update your iPod, the previous “snapshot” is replaced by the new playlist.
For example, suppose you have a smart playlist that is dynamic and plays the 50 songs you have played most. As you listen to songs in iTunes, the contents of that playlist change to reflect the songs you have listened to. When that playlist is moved to the iPod, it contains the songs as they were in the playlist when you performed the update. The playlist on the iPod will remain unchanged until you perform the next update. At that time, if the contents of the playlist have changed, the revised playlist will replace the one currently stored on the iPod.
The same principle applies when you make changes to a playlist manually, too. For example, if you sort a playlist to change the order in which songs play, that order will be reflected in the playlist when you update it onto your iPod. If you change the order of the songs in the playlist again in iTunes, the next time you update the iPod, the songs will play in the new order on the iPod.
When iTunes moves a playlist from its Library onto an iPod, it moves only the songs in that playlist onto the iPod. This can sometimes be confusing. For example, if you purchase an album by a specific artist and then include only some of the songs on that album in a playlist that gets moved to an iPod, only those songs by that artist in the playlist get moved onto the iPod. As an example, this can be confusing the first time you browse your iPod by artist and can’t figure out why a song you know you have by that artist is not on your iPod.