That might be because it’s difficult to source reverse log pots. Did they use a normal log putting it backwards instead? This allows for a useful taper with a much more easy to find pot…
I wonder what's the distortion source is.
Is it Q3 and Q4 or is the transconductance amp IC8B is somehow overloading? I'm not that familiar with the transconductance stuff to see it from the schematics directly.