Good question. What you describe is a result of the shift to DVD discs. You see, the DVD holds so much content that many times a disc is revised but the change is not needed for your library. For example a DVD may contain information for all Beech King Air series aircraft and you have the Beech C90 library. For this example lets say the Beech 300 IPC is revised on the DVD. You will receive a new DVD but after inserting the Yellow update disc you will not be prompted to insert the Beech DVD because none of the changes on the DVD are needed in your library. So why did you receive the DVD? Because NavigatorV always knows and uses the most current DVD you would need this DVD in case of reinstall, you buy a new PC or other any other reason where you may need the base content.
To summarize, there will be many times you receive a DVD and the update does not ask for it. Just file the DVD away as a backup. The update process knows what documents were added or revised and will prompt for only the discs needed.