Hopefully I can explain this without being too confusing. I have a number of smart playlists in iTunes, and many of my songs are on more than one smart playlist. I have only selected certain playlists to be synched, and unchecked playlists are showing up on my iPhone, but only with the songs from the larger playlist. Hopefully this makes sense, and someone can tell me if they're experiencing the same thing. Sounds like a bug to me.