If only one page shows on the Usage them, then likely the other page in question is deleted.

To confirm:
  1. Navigate to Site Explorer>Pages & Templates
  2. Under Folders, select Deleted Pages/Templates
  3. To verify if the page you see does actually include the part in question, mark the button next to the page, and click Restore. Make note of the page name
  4. After the page is restored, click the Search page and search for the restored page
  5. After you locate the restored page, select it
  6. Review the lower content pane, does the part in question show?  If not, repeat the above steps for any other remaining pages until you locate the part in question
  7. After you locate the additional page with the part, you may cut the part from the page and delete the page