it’s because when a ball is released, it accelerates along the vertical axis, but moves at CONSTANT VELOCITY along the horizontal axes,
the camera is aligned so that we can only see horizontal motion, and is sufficiently far that the balls don’t measurably change size as they move towards or away from the camera
so what we are seeing is PURE CONSTANT VELOCITY MOTION.
as soon as a ball leaves her hands, they move with constant velocity
and the grid lines of the street below make it even more apparent