This is one issue with saving a Word doc as a filtered web page. It styles everything, and includes all styles in the document, whether they are used or not.
Word doesn't create a css file, it puts the css styles in the head of the document.
In Word I make sure that the styles I want in the final book are set up and used.
When I import the html file into Sigil, the first thing I do is delete everything within the style tags in the head. Then replace <p class="MsoNormal"> with <p>. The style sheet used in Sigil should match the styles in the Word doc. Now that stylesheet will control the styles and text in the file you imported.
Bob