Warren in Lancaster, PA
I think the flat back people mostly did not have much experience. If you start with a flat back and use the chisel a lot, then they tend to get a little bowed. The reason is that as we sharpen we only polish the back near the edge. If we do this a few thousand times, there is more wear near the edge and so it gets bowed. Nobody wants to work the whole back every time the sharpen, and nobody wants to flatten the whole back again every year or so.