I know you don't see it. This is not something obvious or intuitive. If you think Word is an intuitive program, you are in for a shock.
Look at the
tutorial I linked earlier. That tutorial shows how things in the header like textboxes, that are invisible, can change placement of text in the document, can even make it like one or two lines have different margins. While the purpose of the tutorial is to help people make letterheads, it serves this purpose well.
Again, if that doesn't help, consider posting a sample with the problem. Here's how to fill a copy of your letter with dummy text.
The rand "virus": or how to insert dummy text into a document by Suzanne Barnhill, MVP
Thinking about it. Here: