I don't think it's really a superhuman deduction. I think if you spent time carefully observing people you're around a lot with the intent of figuring out how they think you'd be able to do something very similar. It's just that most of us (including me) don't do that very often. Most people are reactive about such things rather than proactive.

As far as userids, neither you or I should be making guesses about people based on such things. :-)