Syncing selected shelves results in only that shelf on the iPhone

I have a question about syncing selected shelves - I keep my entire library on my iPhone and it works very well. However, if I add a few movies (or books or gadgets, or whatever) and then try to sync only that particular shelf, then that shelf is the only one that is displayed on my iPhone. All other shelves are deleted from the app. I tried to look at the help section, but that doesn't exist yet. :)

Is this expected behavior? It seems that it would be counter-productive (especially with some larger libraries) to have to sync an entire library in order to only add a few items. Is this a bug? Or if this is what is supposed to happen, let me know and I will merrily go on my way singing the praised of DL2! Any help would be appreciated. Thanks.