I think there is a change to the net spring rate when the shock is in a 'lay down' setup like this, due to the change in geometry. Think of it like this: with the shock in a vertical orientation, with the shock at 90 degrees to the wheel travel, an inch of suspension travel nets an inch of shock and spring compression/travel. At 45 degrees, you'd get half the shock compression given the same amount of wheel travel. You could obviously tune for this if the shock was designed for less travel for a given amount of suspension movement, but I think this is harder to optimize with such little travel.
Disclaimer: I'm not sure I'm right, this is my understanding of the effects on the geometry at work here.