I think I may have noticed it because my keyboard doesn't actually HAVE page up and page down keys, so I press Fn-Up for Page Up, and Fn-Down for Page Down. So I'm actually using the same arrow keys for both sets of shortcuts. But even for those who use the page up and page down keys, I think it is an inconsistent system.
I know for me, UP intuitively means backwards and DOWN means forward, whereas I asked my friend and she felt the opposite, that DOWN means backwards and UP means forwards. So WHICH set of shortcuts would be better to change is definitely going to vary from person to person, but even just having the two sets be standardized with each other would make it easier to remember!