Having trouble thinking this through.
Can you have level frame/level ground and uneven springs ? Shouldn't a level frame require level ground AND even springs?
I would think either ground is not truly level and corrected by uneven springs, or ground is level/frame is bent and corrected by uneven springs, or... ?
It just seems that frame couldn't be level without Both ground/springs being good, or equally bad. Or possibly if tire size/pressure/ load offset something.
And you have a single d-side zero rate that something is offsetting if your frame is level.