Marzocchi PM Variable?

[DKWRACER]

Another subject for discussion. NO not an Italian female with hormonal problems [8D]

In my rebuild process, and one for us, is a simple question. What causes the stiffer rebound characteristics within the shim stack of
  -2 to -4 designations? Have dissasembled several sets, but I missed something[)]
Still wondering why?
Thanks in advance!
Tom Brosius

[Rocket]

Tom
I believe it has to be the thickness of the shims, that's the only difference I know.
Rod