Technically, Nerdanel met a grand total of one human, three hobbits and a dwarf before the train, so although Tuor certainly is exceptional, it's more that she... just hasn't really met any.

She gasps in horror. "But that's terrible! Whyever would they do such a thing! Surely you are not at war?" Which is literally the only context she can think of where such horrible things happen. Well that and kinslaying.