Banshee tends to sync all available local playlists to a device, even though you only tell Banshee to sync the device with e.g. a selected playlist.
I consider this unexpected, personally even unwanted, incorrect behaviour.

In my case the problems occur in combination with an iPod, but I expect the behaviour to be the same for other devices.

Steps to reproduce:
- Create/have two music playlists in you library
- Remove all playlists form a connected device
- Sync the device with one of the library playlists by selecting that playlist in the device Sync Preferences

Result:
- Two playlists will be present on the device.

Expected results:
This is a matter of preference.
Personally I asked Banshee to sync the music FROM a selected playlist. Only the music WITHIN that playlist should be transferred. Neither the selected playlist to sync from nor the second playlist should be present on the device afterwards.
One could argue that at least the selected playlist should be added to the device but then again you are not really asking for it.

The only justification for adding all playlists to the device is when "Sync Entire Library" is selected. If one wants a playlist from the library added to a device one could either create one manually on the device or e.g. drag one from the library to the device.