This is because the paragraph or portions of the paragraph have been formatted with additional text styles. An example would be highlighting a paragraph then applying a new font to the paragraph. Because, the new font was applied independently of the paragraph style, it will remain when changing the new paragraph style. This prevents changing paragraph styles from losing any formatting that was applied to portions of the paragraph such as a word that is bolded.
If you have wish to have the paragraph changed completely to the new paragraph style simply re-select the paragraph style in the drop down list in the toolbar.
Tag: Outline 4D, StoryView