This is probably the best video I've seen, and now I see what people are talking about. The smooth scrolling is a result of inertial scrolling.
If you use a trackpad, and give the window a quick swipe, it will continue to move slowly eventually coming to rest. When you press and hold an arrow key on the keyboard, the system knows you want to continue scrolling, so it processes the scroll smoothly.
When you use a mouse with a discrete scroll wheel, it likely needs a driver that tells the system if the wheel scrolls at >N clicks per second that it needs to activate inertial scrolling. That doesn't seem to happen and so it treats scrolling with the scroll wheel like scrolling with a series of independent taps on the arrow keys.
As you know Apple tends to like to promote "lock-in" so it doesn't surprise me that they built the interface to respond more effectively to the swipe gestures used on their first party input devices. While I am sure that they could write a generic driver that fixes the problem, I doubt they will because they do not have any competing first party devices that would benefit from it. Their solution with the Mighty Mouse years ago was to have its scroll ball click to update changes in position with a higher resolution than most 3rd party scroll wheels.
