North and South Holland are two of the provinces of the Netherlands along the west coast. When commercial trade and travel between the two countries became widespread in the 16th and 17th centuries, many of the sailors would say that they came "from Holland" (meaning the coastal regions) and were misunderstood to mean from the whole country generally. The name somehow stuck and became a nickname for the whole country.
A similar thing happened with Persia / Iran (Persia is a region of Iran - and ahppened to be where most of the trade with the western world took place), and of course outside the UK a lot of people refer to Britain or the UK as "England".