I would suspect that in certain instances, amp and amp envelope have a ratio type interaction with certain other parameters. At a minimum amp envelope is shown to impact the amp parameter in the voice signal flow chart and we know that level does not directly interact with any parameter but will drive the signal as described by humanprogram.
One would reasonably have to assume that this interaction of putting amp/amp envelope post filter and filter envelope and before pan creates an interaction of parameters prior to the introduction of FX.
I don’t have hard evidence to this effect, I’m just going off of what’s shown in print and my assumptions about their algorithms.
To be more specific, level should cause a linear increase in volume whereas I’d assume amp / amp envelope are actually interacting with or altering the shape of that which enters into it before stereo distribution.