If I am not mistaken, Initially anarchism was called libertarianism, but later libertarianism was co-opted by American pro-market believers and now refers to anarcho-capitalists.

As for why an-caps are not real anarchists you can read an in depth explanation here. But briefly it is just an oxymoron. Anarchists believe in a world without hierarchies and capitalism is a system defined by the existence of two classes of people(capitalists and working people) and their relationship to the means of production. So there is an inherently hierarchical relationship within capitalism.