It is in the header, even if not in the header area. The headers and footers are a separate layer in the document and that layer includes the entire page. It must have been inserted when you were editing a header or footer.

Sections / Headers and Footers in Microsoft Word 2007-2013

If you actually want it in the header, so that it will be less likely to be moved and changed on the first page by mistake, put it in a first-page header. If you want users to be able to easily move or delete it, cut it from the header and put it in the body of the document.