When the cannonball is fired at low speed, it moves forward and down. Earth gravity has a component in the direction of the ball's motion, does positive work, and speeds the ball up. At a high enough speed, the cannonball's falling distance matches Earth's curvature so that the cannonball always moves parallel to the surface, at right angles to gravity. When a force acts perpendicular to an object's direction of motion, it does no work and does not change its KE.
Work Step by Step
This is discussed on pages 190-191.