On a normal beam the distance from the center of the spring perpendicular to the swing axis (the axis through both rose joints) is roughly 31 cm. And the distance from the center of the hub to the swing axis is 49 cm. So the motion ratio for the standard beam is roughly 31/49 = 0.63.
For the 6 degree beam the swing axis is moved and the distance from the spring center is now close to 35.5 cm, while the distance from the hub to the swing axis is about 48.5 cm. This makes the motion ratio 35.5 / 48.5 = 0.73.
So the spring is not getting closer to the wheel, but it's getting further away from the swing axis, which effectively makes it stiffer at the wheel.