I think it's more than possible that someone might have a skewed perception of their own type yet still use sound reasoning to type others. I mean, someone might decide they need a typing, uh, readjustment to be a special someone's dual . That has nothing to do with how they would type other people. What first comes to mind, though, is the tendency for people to use different methods for evaluating other people than they do for themselves. Here's a excerpt from a wiki article:

"...when people decide whether someone else is biased, they use overt behaviour. On the other hand, when assessing whether or not they themselves are biased, people look inward, searching their own thoughts and feelings for biased motives."

That's not to say evaluating type is the same as determining bias. But, as we aren't privy to other people's thought processes, we need to use concrete indications that might clue in on their type. Content of posts, how they describe their life experiences, interactions with other forum members, etc etc. While when we are evaluating our own type, we do have that perspective. Then our ability to type ourselves has to do with our own self-awareness. And we'd be subject to all sorts of other things that aren't present when trying to type other people such as wishful thinking. All sorts of reasons why someone's self-typing might be unreliable but they could be fine with typing other people.