With Japanese tools, a big deal is made over flatness on the stones, especially for plane blades.

Any out of flatness is registered on the blade as you work up through the grits- an even polish on one stone will often show as slightly convex on the next finer one. Getting the polish out to the edges on a 10k stone after the 8k one (the 10k only occasionally for show/vanity really) cannot be more than a few microns. In spite of trying to use the surface evenly, I don't recall ever seeing any be convex, but that's only over 40+ years.