That's a problem mostly caused by using passive log-taper circuits, instead of alternate active volume control circuits that use linear-taper pots, which have sufficiently good tolerance and ganging in low-cost dual units. But these circuits require 1-2 op amps per channel and those 20 cents are too much in three-figure hardware.

Ganged log-taper pots have something like +-3 or +-6 dB ganging mismatch even for relatively high-end (10-15 € ea) units.