It's projection. Not the "I'm ascribing my attitudes to the other" flavor of projection that most people think of. It's the kind that makes someone go "Oh, of course this other has this set of attributes that make them exactly the kind of person I already have a stereotype for and opinion of."
Secondly, there are people that spend a great deal of mental effort concealing what they consider to be their "true personality." Whether or not that is in reality who they are is tangential to the point that concealment becomes a big part of their life, and they start assuming others do it to. Or if someone was burned by a person that practices concealment, they'll be on guard for it. And that is again tangential to either's _ability_ to read a concealed aspect of another, which are often no closer than the projection described above.
Secondly, there are people that spend a great deal of mental effort concealing what they consider to be their "true personality." Whether or not that is in reality who they are is tangential to the point that concealment becomes a big part of their life, and they start assuming others do it to. Or if someone was burned by a person that practices concealment, they'll be on guard for it. And that is again tangential to either's _ability_ to read a concealed aspect of another, which are often no closer than the projection described above.