When you create an event in iPhone stock calendar application, you can choose in which calendar you want to create it. But only calendars selected to be synchronized in BirdieSync options in Thunderbird or Sunbird are synchronized (menu "Tools/BirdieSync/Options...", "Events" panel, "Calendar" tab). It means that if you create a new event in the "Home " calendar on the iPhone and that only "Work" calendar is selected to be synchronized, this event will not be synchronized.