I think what this thread is really trying to get at, ultimately, is how does one differentiate between someone with NPD and someone who does not have NPD but who exhibits some noticeable level of Narcissism.
The answer to that question is actually quite simple. In the Non-NPD with Narcissism, the Narcissism is usually the result of some accomplishment, achievement, or life event as a young adult or adult. For example, someone who grows up poor or middle class, has a natural talent for singing, and goes on to become a pop star earning 4 million dollars annually is also someone prone to developing Narcissistic traits, even though they were a perfectly normal neurotypical before all of this happened. They might ditch their old friends, use women as sex objects, buy only the finest things, associate only with other high status people, etc. Everyone then looks at this person and thinks that they are a pwNPD, when, in fact, they are just a regular person who became Narcissistic as the result of a life event that gave them a big head.
Similarly, you could also have a 24 year old girl, who knows she is in her sexual prime, who wasn't particularly attractive at age 14, but who has finally come into her own, and who realizes she now gets a lot of male attention that she never got before. She may develop compensatory Narcissism, and begin to wear eye-catching clothing and refuse to talk to men she views as beneath her. Again, not NPD, just a Non whose head got big, and who is maybe compensating for secretly feeling that she's still the 14 year old girl that no one noticed.
Someone with NPD, however, has a Narcissism that is either primary or that began so early in life that it takes years of self-reflection and/or therapy to dig out what the reasons might be. So the grandiosity in the person with NPD isn't the result of an achievement, accomplishment, or big life change. It's just there. And the achievements, accomplishments, etc, of the person with NPD will simply serve as evidence for the pwNPD of his already-existent grandiosity, while with Nons with Narcissism, those achievements, etc, are the cause of their Narcissism. That's why someone with NPD who makes 40,000 dollars a year will score higher on the NPI than a celebrity who makes 4 million dollars a year. The pwNPD's grandiosity is not something that became added to their sense of self as the result of becoming rich, famous, attractive, or successful. The pwNPD's grandiosity IS that person's sense of self. And it generally has been for as long as that person can remember.