Channel Imbalance At Very Low Volume

Here's a graphical illustration of an audio volume control's attenuation:

Note that at 0.5% of the rotation (12 o'clock), the attenuation of the pot is actually at 90% resistance. As you can imagine, from 6 to 9 o'clock, the pot is only responsible for approx. 1% of the attenuation range (i.e. 99% of resistance of the pot). Tolerance wise, it's not achievable to be 0.5dB accurate between channels, since a small difference would have resulted in 2-3dB of mismatch easily.