D70 auto focus question

Hi all:
In Continuous-Servo auto focus with the Dynamic Area AF set,
(in manual or aperture priority) when I hold my hand in front of the
lens and acquire focus, then slowly move my hand to the left or right
(still with the shutter button pressed halfway down), the lens will
simply focus on whatever is behind my hand after I've moved it.
Shouldn't it stay focused on my hand? In the manual on page
69 it says, "...camera tracks subject as it moves from one focus area
to the next." and on page 140 in referring to Dynamic focus, it
says"focus area selected in viewfinder does not change."
But I did notice that when I shift to the auto sports mode,
the camera does hold focus on my hand as I move it around. So,
shouldn't that be the way it acts all the time when I am in Continuous
Servo auto focus with Dynamic Area on, in manual or shutter priority?
Please clue me in on this. Thanks. Ken