Well I don't think people have as much "clean" access to their own behaviour and thoughts, as is generally thought. By thinking about our thoughts, we alter them. So that's why no-one can be crystal clear about their own motivations.
I don't confess to feigning surprise in my comment. I believe it is very common, purely from observation of others - especially children and young adults.
OK, you don't apply Bayesian priors to other's behavior based on your own internal knowledge. I'm not sure why you have much confidence in the analysis, then. See the second argument of chroma.
What's a Bayesian prior?
I guess they mean "how likely it is that they know something just because you know it".
I agree with the GP. I find myself surprised at something someone didn't know, only to realize upon further reflection that I wasn't actually surprised, I was just asserting superiority, which was unconscious and crappy.
It changed after I saw the XKCD "Ten thousand" comic, which changed my worldview for the better.
The best argument you have, from your experience, is that you often feign surprise, which I find a bit damning, if you then ascribe that behavior to others. I can't remember feigning surprise, but I definitely remember being horrified at being forced to argue about something rather elementary. If it doesn't affect me directly, I'm more likely to simply ignore the person's argument and go my way.
edit: to be clear I get no satisfaction in seeing other people make obvious mistakes... it's almost physically painful.