If a friend said the opposite about themselves you would know instantly it was untrue, and you would be able to say why at length, with examples, without pausing.
Run the same audit on yourself and a different answer comes back, and it never occurs to anyone that both results cannot be right.
The test people apply to themselves is one they would never apply to another person, and would not accept from one either. Nobody has to qualify. There are no entry requirements, however convincingly the feeling argues its case at four in the afternoon.