As I've read there are some types of undead that can be frightened and some types which cannot be frightened (see here Can undead be frightened? for reference).
The list which undead can be frightened and which not is quite counter intuitive.
Zombies, Skeletons, and Wraiths can be frightened. Ghosts, Shadows, and Mummies cannot be (to me there seems to be no clear separation there as the most automaton like can be frightened, but also some not so automaton like. Also its not all corporeal that can be frightened, and not all non corporeal that cannot be frightened).
I find the list counter intuitive because I would have expected the list to either contain only low level undead that can be frightened OR that it contains only intelligent undead (thus no automatons as automatons normally don't have the mental faculty expected to feel fear, while intelligent undead should have that). But from what I can see it encompasses different levels of difficulty of undead AND also automatons and non automatons.
My question here is, is there anywhere mentioned WHY the list is as it is? Thus is there any reason in the rules or in the fluff text for the undead or fear, or any commentary from developers, that gives any indication to why this list is so counter intuitive?