Narcissists do regular human immaturities but with the volume turned up, and they don't think it's wrong. 
 ~ Like believing they know all about a person and that person's life, 
past and present; abilities, preferences, fears, motives, habits, and 
LACK of ability, experience or knowledge.  
 They actually think they
 know what another person's "limitations" are, and what they do with 
their time, and what they have or have not accomplished, and why they do things.
(Like all the people who assume they know why I write this blog, who have never actually asked me the question, or who have asked the question, but simply dismissed my reply (or even argued with me!) because they weren't actually interested in the answer, lol. Poor dears. ) 
 If they 
know the person, it's usually worse; a Narcissist will build a whole 
story around another person, and simply treat the person according to 
this fictional story, forever. They will not doubt themselves, they will
 not try to find out more about the person, they will literally ASSUME 
they know all about the person, as if the person's life and being could 
be written on a one-page biography and read in 10 minutes.
 Their 
over-confidence in their assumptions and assessment of others is truly 
astounding, and the lack of ability to take in real information about 
others (or about themselves) causes them live in a fantasy world. They 
completely trust their own assessments about everyone and everything, 
they think that whatever they THINK they see IS what there is. No need 
to find out if it's true or not... they already know... and if you tell 
them something different, they will simply say you're lying, or you 
don't know what you're talking about.