I think these "anarcho-capitalists" adhere to a convenient vulgar simplistic definition of anarchism. Mainly I think their definition of anarchism is "opposition to the state" when anarchism has meant opposition to authority and hierarchy in general and not just bureaucracy, politicians and other components of the state. Wage work and having bosses are obvious forms of hierarchy and authority and they are main features of capitalism and it is a main reason why most anarchists don“t include "anarcho-capitalism" within anarchism alongside the obvious fact that anarchism has been historically alongside marxism a furious anti-capitalist position.
But also I think "anarcho-capitalists" are conscious entrists (see wikipedia article on the political phenomenon called "Entryism") inside anarchism who want to parasite its history and in the meantime also get a certain rebel edge which comes with the word anarchism. And so for example they have spent time on ridiculous maneuvers in places like wikipedia where they have tried to appear as an important part of anarchism mainly through obscuring anarchism history and through trick conceptualizations.
Since "anarcho-capitalism" adheres to a kind of neo-liberal economics called "austrian economics" it is clear that they are just a more radical form of neoliberalism, and the mainstream form of neoliberal ideology being mainly what is known as "minarchism". In real activism and debate they spend their time alongside conservatives and so called minarchists but nevertheless somehow decide to call themselves "anarchists". It seems that this "anarcho-capitalism" phenomenon anyway is mostly a USA thing and so anarchists outside the US like me became aware of this thing through the Internet but it looks like in the USA they exist within the neoliberal "Libertarian Party" which is a party which has people who also act in the right wing conservative Republican Party. This shows "anarcho-capitalism" is clearly a radical section of the neo-liberal right wing which means it has almost no relationship with anarchism and anarchists and so even a famous US anarchist like Bob Black has said that these "anarcho" capitalists "seem to have no noticeable presence except in the United States, and even there they have little dialog with, and less influence over the rest of us."
http://www.theanarchistlibrary.org/HTML/Bob_Black__Theses_on_Anarchism_After_Post-Modernism.html
From what I know "anarcho-capitalism" is not the only contemporary case of strange mixing of strongly contradictory ideologies. In a somewhat similar case in Russia there are some guys who came up with a thing they call "National Bolshevism" apparently trying to mix marxist-leninism and fascism. It seems to me this phenomenon just as the USA "anarcho-capitalism" thing obey to specific historical and local political situations.