It was filed in 2005. I guess that someone thought they could. It might work well in their Office products as well. ;)

An interesting comment from Microsoft on this article [news.office-watch.com]:

Microsoft is certainly not trying to patent the "page up/page down" function on a keyboard. Instead, our company’s technologists and patent attorneys (and now the USPTO) believe we’ve come up with a way to improve overall user experience. This patent describes specific, new techniques for incrementally navigating through paginated content, making it easier for users to navigate through documents. For example, it includes a particular technique of calculating a point offset from the top, which offset is used to permit scrolling in a whole-page increment to a another page displayed at the same offset.