Those are precision ground. Note the shinyness and very smooth, backlash free motion when you rotate the nut on the screw. If they were rolled, you'd see a small groove between ball grooves as a result of the metal being pushed out of the ball grooves. You'd also be able to feel a slight amount of wiggle.
For an example of when two screws are in good use see:
http://www.cnczone.com/forums/showthread.php?t=34954.