Basically, the "social" definition of introversion and extroversion, would be what in my discussions is called "expressed Inclusion". That determines how much you approach others for interaction on a social level.
If a person is an "introvert or extravert" (only extravert has the "a") in the Jungian sense, of dominant function orientation, then as has been said, that's about where their energies are focused. Yet, it just so happens, that where their energies are focused will correspond to the other version of e/i, in how much they approach others socially. It makes sense that someone whose energies are directed outward will tend to approach others, and someone whose energies are focused inward will tend to approach others less.
There are other factors that can help shape interaction. Such as wanted Inclusion, which is basically people vs task focus, and basically integrated into the type system as "directing/informing" (which is actually one aspect of the dimension). In contrast to "expressing", this has been described as "responding as an introvert or extrovert". It's how much a person wants to be approached by others, socially. So that may explain why some who are quick to approach others might be slow to respond, and vice versa.
There's also even Keirsey's pragmatic/cooperative, which I believe is a factor known as expressed Control. Like i/e, it ultimately tells you how much a person approaches others, but this time, in leadership and responsibilities, instead of social interaction. A person who "does what works" will be faster to take action than one who must "do what's right". And then structure/motive, like directing/informing, is another level of responsiveness.
So if EN's, or ENP's are said to be "introverted extraverts"; while expressive on a surface level, they might be cooperative (ENF), directive (ENJ), structure focused (ENT), and thus not as 'connecting' with people on either the expressive or responsive dimensions as you might expect for an "extravert". They would them seem more "internal" in some way, or see themselves that way.
Also, personal experience can affect a person, and make them less expressive. In such case, it is just a typical aspect of their type they are deviating from. For whatever EN's/ENP's that might seem more reserved; there are plenty that fit the common model of "outgoing, bright, gregarious", etc. So "introverted extravert" is not really any hardwired type trait.