Jim,
Pure speculation on my end, but I suspect that the rounded (convex) bevels on those chisels is the result of frequent sharpening freehand, likely on the job site, more so than a desired feature, I may be wrong. It is not an exaggerated curve, just off from flat. Being on the convex instead of the concave side also supports the "better edge retention". Levering against the bevel, would also seem to be better with a convex bevel.
/p