Strange - this would more likely be a pot issue. Pots can be quite variable in relative change for a given rotation at the very lowest end of their scale. It is quite possible that accuphase C200 came with different pots throughout their manufacturing life or an old pot may have been replaced at one stage (these things are mechanical so eventually they get dirty and are no longer smooth so they tend to be replaced).
Gain Pop....causes??
I have had several Accuphase C200 preamps...my experience is my normal listening level on the volume control is 10 o'clock or 10:30...this has been the same with 3 different C200's. I have used the same amps. I have had 2 preamps upgraded and recapped...a nichicon version the gain dropped a little to about 9:30...but a Black gate version dropped all the way down to 8:30...now 10:00 would blow me out of the room. Has anybody experienced this with cap upgrades?