A family of Microsoft word processing software products for creating web, email, and print documents.
If you are editing the document in sharepoint, chances are good that you are using the online version of Word. This is a different program.
Rather than relying on what you see in the app, what do you get if you print or produce a pdf? Is it still different.
When you click the option to update the style to match selection, you are changing the style.
Differences between using a document in the browser and in Word