Scrolling: to proceed, move fingers up on touch devices, but down on trackpads. Why?

[2011-01-25] hci, computers
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Category  “intriguing useless facts”: On Macs, you can use two fingers to scroll an area. But then you move your fingers in the opposite direction that you would on a touch device. Attempt at an explanation:
A touch device obviously simulates how you would move a scroll of paper in reality. With a trackpad, I think the metaphor is “moving the scrollbar”. If you were to single-click and drag it, you would make the same movement as you do with double finger scrolling.