Why do scrollbars move in the opposite direction of the page movement?

Since getting my tablet PC, I've been using a couple Firefox plug-ins that let me drag the page (Grab and Drag) or quickly page up/down (All-in-One gestures). Grag and Drag lets you drag a page with your mouse much like Adobe Acrobat does.

What I quickly realized is that when you drag a page, your mouse motion is the opposite direction compared to using the scrollbar. That is, dragging a page up is like dragging the scrollbar DOWN. I suppose that the scrollbar is essentially positioning a viewing “window” on the page; move the window down and contents move up. When you drag the page, you are more or less dragging the contents, not the window.

Now my dilemma: Right now, I have All-in-One gestures set so it is more akin to dragging the page, so if I right-drag up, it does a page up; right-drag down, and i get a page down. Typically, I think you would have this mimicking operating the scrollbar so dragging down would actually trigger a page up.

The reason I did this was that with my tablet, there is no physical right mouse button. It imitates a right-click, if you hold the pen stationary in one spot for a moment. When I had things set up reverse of what I have now, that is, right-dragging up did a page down, here's what would happen. I would be dragging a page with a left mouse drag, so move up and the page moved up. HOWEVER, if I accidentally held the pen too long in one place, it would switch to a right-drag. Well, a right-drag up caused a page down to occur and the page moved down!

The only point of confusion now–for me at least–is that it's the opposite motion of the mouse scrollwheel. Oh well I guess my brain will get used to this.

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