As far as I know it's not known exactly, but I can give you a few facts about Holland.
- Contrary to popular believe, Holland is NOT derived from "Het Holle Land" (meaning the Hollow Land), which could be a reference to the Netherlands, and thus also Holland being below the sea level. It comes from "Holt Land", and "Holt" means "wood" in ancient Dutch (no longer in use today, so only a few souls know that).
- Most tourists visiting the Netherlands never come any further than Amsterdam, Rotterdam or The Hague. These cities are all in Holland. Amsterdam is the capital of the Netherlands and the government is in The Hague.
- Most importantly, during the VOC time and the Golden Age, it was most of all Holland that benefited most.
- De Randstad, the heart of the Dutch economy is most of all in Holland (and for a part in the province of Utrecht. Purple area in the center)
- And (sad but true), a lot of areas outside of Holland (particularly the border with Germany) are quite often hardly ever mentioned, as well inside as outside of the Netherlands. Many attempts have been done though to change that, but none appeared successful.
- And even the Dutch themselves are not in consensus here. It's a geographical fact that Holland is only a part of the Netherlands, but not all of the Dutch care (much to the chagrin of those who do).
Nothing against Holland, but I'm not from there. But hey nothing against Ireland either, but I am not from Ireland either. (My former wife was from Holland. I may be divorced now, but if I had something against Holland I'd never marry her in the first place). Some of the facts I mentioned above may have played a role in this confusion.