[W126 Coupe] Suspension Leveling

[Sam Sisic]

Hallo group,

I understand that sometimes it is required to use different size rubber
shims to level the car at all four corners or equal height left to
right, AKA "gangsta lean". 
I would like to understand what is being compensated for with this.
Is it a tolerance in the chassis/suspension components?
Is it the coil spring rate variation/tolerance?
Is it compensating for worn bushings, in which case those should be
replaced first?
I will need to do this to mine and will need some 4-nib shims to do it
if anyone has a lead on those, I would appreciate it. I checked with the
MB stealerships and they only go to 3-nib. I know the 4-nib exist as
they came off the front of my buddy's car when he had the springs
replaced with H&R's. Had I known, I would have grabbed them before they
ended up in the trash.
Thanks,

Sam