It's definetly a mistake. I suspect it's got more to do with taking prints off old plans that may have shrunk due to exposure to moisture or that were photo copied and suffered some sort of distortion in the copy process.

It has to be a mistake, at least in the vast majority of cases, because in many cases the old plans only showed you 1/2 a wing along with squares to aid blowing up curved portions and measurements for rib spacing and span. Nowhere in any of the articles I've seen does it talk about assymetrical wing construction.

The flight trim is always set up using warps or drag tabs and control tab offsets combined with thrust offset angles.