It's not unusual with the stock potentiometer. The resistance in both sections of the volume pot aren't always equal which manifests itself as a channel imbalance at low volume levels. If the left channel is coming up in volume and seems even with the right channel at listening levels below which you listen to your music, you don't have to worry about it. If the channel imbalance exists at a level at which you listen, then the fix in the FAQ that Paul mentions will cure it.
Otherwise, if the imbalance drives you nuts, you could replace the stock pot with a "better" volume pot which may (probably will) cure the imbalance issue. I use an Alps "Blue Velvet" (Alps RK27) which is pretty well balanced. It would probably be considered a budget "upgrade" (cost about $10 - $15) but some folks don't favor the sonics of conductive plastic pots, or, at least they don't favor the sonics of the Alps pot. Honestly, I can't really hear the difference. I just like the Alps pots because they aren't expensive and they are smooth turning and generally well balanced.
