Kees,
Do you hone using magnification before and after? One thing I have noticed is that the nose and the heel of almost every razor I've ever examined are not honed as well as the middle of the blade. You may just be noticing something that's always been there! Even if the nose and heel were wearing differentially it may not be the stone or the X-pattern. It may be due to the hardness of the steel being different out at the ends. Why? Different thicknesses of steel cool differently and end up being softer or harder. Not much but just enough. This phenomenon has been the subject of much discussion among the bladesmith community. Just a possibility to think about in case there weren't enough variables in the equation....