I suspect they're very closely linked in some people. I wonder if in years to come we'll gain a much better understanding of these things and perhaps realise that we're simply describing different sides of the same coin. Certainly, I think the DSM classifications need to evolve and change (much as the ICD system has) as the DSM's personality disorder categorisations are too reductive and in some cases quite arbitrary. It's interesting to note though that conduct disorder is so closely linked to antisocial personality disorder (another cluster B personality disorder, which can look quite similar to narcissistic personality disorder in some aspects).