First, for new documents, you should be creating them from templates, not using old documents. Because of meta data issues, this is especially important in law firms.

Any chance your problem documents have the option checked under the attached template dialog to update styles from the template? You do NOT want this checked.

Is this happening on one computer only or on multiple computers?

Here are two primary references on Styles, neither of which will directly answer your question:
Understanding Styles in Microsoft Word
Tips for Understanding Styles in Word by Shauna Kelly