One possible explanation:
When you place non-global fields in either the header or the footer, they show data from the current record while in browse mode, but in Preview or when printing/saving as PDF, fields in the header show data from the first record on that page and fields in the footer show data from the last record on that page.
Thus, you can get differences in what data appears when you leave Browse mode and preview such a layout. (Preview mode also automatically previews the first record in your found set--which may not be the same record as the one current when you left Browse mode for Preview.)
Fixed with a global field. :-)