That means that the chapter mark did not fall on an "I" frame. When this happens, you will find that when you search chapters, the chapter may be off up to 10 frames. DVDA will adjust the chapter marker to fall on an "I" frame.  To avoid this, either shift the mark to an "I" frame within DVDA (when you start moving the marker, you will see an indication of the nearest "I" frame). Of if you use Vegas to create chapter marks, select "insert I-frames at markers" on the MPEG options page. This will guarantee your markers will be on the exact frames that you want.