TeX's model of the world is (broadly speaking) that the author writes text, and TeX and its macros decide how it all fits on the page. This is not good news for the author who has, from whatever source, a requirement that certain things go in exactly the right place on the page.
Fortunately, in the LaTeX world at least, there is a fixed point on every page, to whit the page header. The package textpos latches bits of typesetting to locations you specify, by fixing them to the page header, and thereby solves the problem.