[kiaulen]

For the same reason that controllers have both "normal" and "inverted". It's all in your point of view. Are you controlling like a mouse (up on the joystick looks up) or like a joystick (forward on the joystick is like tilting your head forward, and looks down).

For me, I hate the "natural" scrolling, because I map scrolling motions to "up" and "down". Moving my fingers downward should do what I think of as "down", which is to see what's below my current screen. I have an XPS 15 with the touchscreen, and only when you're actually touching the screen does "down moves the document down (and goes up toward the top)" make sense.