I don't know if anyone has discussed this before, but I think alot of confusion over whether someone is an ENTP or an INTP has to do with the reason why they have been isolated from society in some way.
Including myself, most ENTPs are expedient people who arn't afraid to challenge social norms and the status quo; they have alot of charm and sociability when they arn't annoying the heck out of people, which could be alot or all the time. I think as a result of this, ENTPs sometimes form an inferiority complex where they believe that people do not like them and that they are not accepted. This will cause someone who would be otherwise and extravert to develope tendencies to compensate for this fact; maybe even an ENTP would even stop trying to associate with society causing them to appear quiet or introverted regardless of the fact that they would rather associate with people as an extravert. Often for this reason they become aloof and detached and be sporatic with people they like, being hot or cold at various times. This is part of the reason I think that many ENTPs test as INTP when actually ENTP fits them better, as indicated by the fact that the description of an ENTP fits me 100% where as an INTP maybe 25% even though I tested origionally as an introvert.
On the other hand, an INTP may have no real reason to be introverted besides the fact that they just are naturally introverts. Maybe society reinforces their introversion, maybe it doesn't. It could just be who they are inside and don't really feel much for society and that suits them just fine.
As for me, I would rather associate with society, but I don't follow societal norms and I am judged because of that, so I stay secluded sometimes to avoid conflict. I hate it, and would rather be with people. But, that is the way it is with people.