I think "Styles in use" means "Styles that have ever been used in this
document (or template)." So if you format a paragraph with Style A and later
change it to Style B or delete the paragraph, so that there is no longer any
text formatted with Style A, Style A will still be displayed as "in use."
You can delete all "Styles in use" except Normal, Default Paragraph Font,
Headings 1-3, and any other heading styles that have been used. Deleting
user-defined styles actually deletes them; deleting built-in styles resets
them to the default Normal.dot default.