Reference aircraft wings and the effect of displacing air around the edge of a surface. The board displaces a considerable amount of air as it falls, some of the displaced air curls around the end of the board with the ball on it (as per the end of a wing), in part aiding in propelling the ball both right (toward the cup) and counteracting some of its ability to fall- giving the impression it moves slower.

Thoguh the 'arch' hypothesis remains true, the effect would be considerably reduced in a vacuum.