I just read one good reply. Holland used to be the most powerfull province of "The Republic of the 7 Netherlands". All provinces had certain autonomy and all the colonisation was done by the province of Holland (with help from Zeeland, 'Sealand'). So foreign countries always had to deal with Holland and not The Republic, which is why still to this very day people speak of Holland.
Now after Napoleon, The Republic and the Hapsburg Colony of Belgium (Austrian territory), a.k.a. the 'Austrian Netherlands' were combined. The power of the Holland province dissappeared. It was a kingdom with two capitols and two seats of government (The Hague and Brussels). Which is why we speak of South-Holland and North-Holland: North Holland would get the capital (Amsterdam) and South-Holland would get the governments' seat (The Hague).
To make sure that no one would be disadvantaged the Dutch decided to called the country "The united kingdom of The Netherlands" Very soon after though, Belgium fought themselves to independency, leaving the country to be named "The kingdom of The Netherlands".
Now, the term 'Dutch' comes from 'Dytschen', because the founding father of The Republic comes from a place which currently lies in Deutschland, better known as Germany. Germany is actually also an incorrect name, but that also dates back 'till before 1870, when Deutschland was not yet a country but (just like the Republic) a union of small states, with Prussia as the mightiest province. In English 'Dutchland' would be more correct for Germany, from an historic point of view.
j/k wit ya lol
