I met many narcissists who were modest - even overly so. This seems to conflict with your observations. How do you reconcile the two?
The "modesty" displayed by narcissists is false. It is mostly and merely verbal. It is couched in flourishing phrases, emphasised to absurdity, repeated unnecessarily - usually to the point of causing gross inconvenience to the listener. The real aim of such behaviour and its subtext are exactly the opposite of common modesty. It is intended either to aggrandise the narcissist or to protect his grandiosity from scrutiny and possible erosion. Such modest outbursts precede inflated, grandiosity-laden statements made by the narcissist and pertaining to fields of human knowledge and activity in which he is sorely lacking. Devoid of systematic and methodical education, the narcissist tries to make do with pompous, or aggressive mannerisms, bombastic announcements, and the unnecessary and wrong usage of professional jargon. He attempts to dazzle his surroundings with apparent "brilliance" and to put possible critics on the defence. Beneath all this he is shallow, devoid of real knowledge, improvising, and fearful of being exposed as deceitful. The narcissist is a conjurer of verbosity, using sleight of mouth rather than sleight of hand. He is ever possessed of the inner sensation that he is really a petty crook about to be unearthed and reviled by society.
This is a horrible feeling to endure and a taxing, onerous way to live. The narcissist has to protect himself from his own intimation, internal on-going trial, guilt feeling and anxiety. One of the more efficacious defence mechanisms is false modesty. The narcissist declares himself unfit, unworthy, lacking, not trained and not (formally) schooled, not objective, cognisant of his own shortcomings and vain. This way, if (rather, when) exposed he could always say: "But I told you so in the first place, haven't I?" False modesty is, thus, a hedging mechanism. The narcissist "insures his bets" by placing a side bet on his own fallibility, weakness, deficiencies and proneness to err.
Yet another function is to extract Narcissistic Supply from the listener. By contrasting a belittling and reducing statement about himself with a brilliant, dazzling display of ingenuity, wit, intellect, knowledge, or beauty - the narcissist intends to secure an adoring, admiring, approving, or applauding protestation from the listener. The person to whom the falsely modest statement is directed is expected to vehemently deny the narcissist's claims: "But, really, you know much more than you pretend to know", or "Why did you say that you are unable to do (this or that)? Truly, you are very gifted at it!" The narcissist then shrugs his shoulders, smirks, blushes and moves uncomfortably from side to side. This was not his intention, he assures his correspondent. He did not mean to fish for compliments (exactly what he did mean to do). He really does not deserve the praise. But the aim has, thus, been achieved: the Narcissistic Supply has been granted and avidly consumed. Despite the narcissist's protestations, he feels much better now.
The narcissist is a dilettante and a charlatan. He glosses over complicated subjects and situations in life. He sails through them powered by shallow acquaintance with rapidly acquired verbal and behavioural vocabularies (which he then promptly proceeds to forget). False modesty is only one of a series of false behaviour patterns. The narcissist is a pathological liar, either implicitly or explicitly. His whole existence is a derivative of a False Self, a deceitful invention and its reflections. With false modesty he seeks to implicate others in his mind games, to co-opt them, to force them to collaborate while making ultimate use of social conventions of conduct. The narcissist, above all, is a shrewd manipulator of human character and its fault lines. He will never admit to this. In this sense he is verily modest.
next: Narcissistic Confinement
Vaknin, S. (2008, November 17). False Modesty, HealthyPlace. Retrieved on 2019, June 26 from https://www.healthyplace.com/personality-disorders/malignant-self-love/false-modesty