The Page Down is executed from the highlighted item. If no item has been highlighted by user it jumps from one page down from first item in list. This behavior is consistent with the arrow keys which also move one up/down from highlighted item.
Testing in Windows Explorer for Advanced Properties and it's behavior makes no sense at all, not sure I have other programs to test what they do in this situation.

1) On e.g. http://img17.imageshack.us/img17/1329/58242939.png, press Page Down

Expected: selection and scroll go down one page.
Observed: both miss their targets http://img801.imageshack.us/img801/3610 ... ongscr.png

EDIT Tracked at http://www.ventismedia.com/mantis/view.php?id=6439