It's been brought up before, but threat analysis isn't always going to be reliable. I remember reading an account from a guy's autobiography; he was in Colombia, coming home from a bar late one night with a friend, when some local people approached them with knives and demanded money. Well, because he and his friend were SAS, they broke the muggers, left them in the gutter, and went home feeling better about themselves.
The orcs could take either role in this equation. They consider themselves hot stuff because they've got big axes and big muscles. They might never suspect someone could beat them, like the muggers; or they might consider themselves to be better than everyone else, like the SAS.
This paradigm, of course, extends to unintelligent creatures, who'll look at the number of legs something's got and judge from there. As far as a dire bear's concerned, if it's six feet tall and on two legs it's just another human. It doesn't have the experience or brains to expect Kal-El.