Throw fails on some mobs

As per Kastellyn's suggestion, I'm posting about it here since it doesn't seem to be a simple misflagged mob issue. Certain mobs, like the dwarf commander in the manor of twilight (the exact name escapes me at the moment, Nalchan or something) have 100% defense rate against throw, despite most certainly NOT being hth specced - the mob in question has spear spec and no hth moves.