Most knives that have been used a lot and hand sharpened over time will have convex edges naturally. Before angle guides and such became popular, folks did not maintain the exact same angle when sharpening. Also, the stones themselves became curved from wear, further curving the edge. Those of you who are old enough may remember grandma's old butcher knife and the long stone that was worn thin in the middle that was used to keep it sharp. Grandma didn't want a new knife with a straight bevel ground edge. It wouldn't cut like her old convexed and slightly recurved favorite knife. We can duplicate years of such sharpening in just a few minutes with a slack belt on a sander. It works.