Most likely, what appears to be one list is really two lists...
If you click the first level 1 list number, Word selects all level 1 items belonging to the the same list level. You can then customize the list via Home tab | Multilevel List | Define New Multilevel List.
However, recreating the numbering from scratch may be safer (although time-consuming). Set it up linked to paragraph styles as discussed in the article at
http://shaunakelly.com/word/numberin...g20072010.html.