Vista - DragDeltaEventsArgs.VerticalChange cumulative

The documentation for DragDeltaEventsArgs.VerticalChange says

"Gets the vertical distance that the mouse has moved, since the previous
DragDelta event, when the user drags the Thumb with the mouse."

To me this means that each time the DragDelta handler is called the previous
value of VerticalChange is cleared out and it should just contain the
differences from the last vertical value. In my app, when the DragDelta
handler is called, VerticalChange shows the cumultive effects of my drag. If
I move the mouse down slowly I get 1, 2, 3, 4.. when I would expect 1, 1, 1,
1. If I move the mouse down and then back up I get 1, 2, 3, 2, 1 when I would
expect 1, 1, 1, -1, -1. I've tried setting args->Handled = true but that has
no affect.

Am I misinterpreting the documentation? Doing something wrong?

John