Artillery was already 'nerfed' a bit to compensate for the game engine causing units to necessarily cluster around action squares. So the effect of artillery more resembles hitting a more dispersed unit.  I just looked, in the CMBN v3.12 patch info there's the line:  Some micro terrain, such as rocky ground, now provides more protection from high explosives. I think that phrase 'some micro terrain' is the key. Terrain elements got massaged up-and-down a bit, some types more than