I think an important factor which nobody has touched on are the forces at play.

If you look up the Veritas Shooting Plane, they advertise that "...it has a bed skewed 20° to reduce cut resistance and make a shearing cut that leaves a clean finish on end grain." The same idea is being incorporated into the ramped shooting board. In both cases, the blade travels in the same direction as the movement of the plane (as you pointed out), and the effective cutting angle is not changed as it would be if it were truly skewed.

However, because the blade contacts the workpiece at an angle to the direction of plane travel, the resultant force is different from the standard shooting board. Instead of all the force being pushed into the fence, a portion of it is directed down into the table. It's this introduction of the downward force that results in a shearing cut. Comparing the force of your hand driving the plane to the resultant force of the blade on the wood shows the true difference between the ramped and traditional boards.