A family of Microsoft word processing software products for creating web, email, and print documents.
When you start a new document, you will not see anything in the Header
However, there is always an empty paragraph in the Header (and footer) of the document
If in your "carefully crafted" document, you had the top or bottom margins set to be less than the distance specified for the Header (or Footer) from the edge of the page, under some circumstances, your margin settings will be over-ridden by the Header and Footer settings from the edge of the page.
If you are going to set the margins to values that are less than the default distance of the Header or Footer from the edge of the page, you need to reduce that distance so that it does not affect the margin.