It could be that the martensite grain size in that area is not as small due to heat treating methods or it could of been overheated in that area causing the grain structure to grow.
the grand cheff line is ground 80/20, you should sharpen asymmetrically by the way. that knife doesnt have very good retention to begin with, youre better off not using a microbevel instead use frequent strop and ceramic honing rod.
Again, thanks all for pitching in on the ideas and info.
If it makes you feel better, I don't usually have any sharpening troubles, but I struggled with the one grand cheff gyuto that I sharpened. The problem I was having looked almost identical to yours and to be honest I don't know that I ever found a solution. Serious wire edge troubles. I'm thinking bum heat treat as the steel felt gummy on the stones. Didn't like it at all.
Don't touch my d!ck. Dont touch my knife. ~ Anthony Bourdain ~