That's right, the gravity is being changed by the device orientation. There's absolutely no other forces being used on the ball.
As the action is currently setup, the ball moves across the screen as it is supposed to (with everything in OnUpdate).
The only problem is the ball jittering as it moves across the screen. Sometimes it's barely noticeable, but other times its as if it has skipped a few frames. The jittering is the type that would normally be associated with a camera moving to follow the player and them not being synced properly, but I've made the camera stationary and the jittering is still there.
I would be tempted to put it down to a complicated scene and a slow processor, but it's currently a very simple scene being used on a very powerful android device. Plus the ball is the only thing in the stage that is jittering. Anything else (such as allowing the camera to move) is smooth.