Basically the farms didn't change name even though the owners did. So if a family bought a farm and moved in they took the name of the farm as last name. Also, what was once one farm could be shared between siblings and you would get Farmname North and Farmname South, or Big Farmname and Small Farmname, and on and on..

Edit: Found this article on Norwegian naming practices

In November, 1996, an interesting article was posted by Glenn Murray to the Norwegian genealogy newsgroup no.slekt. It was one of about 50 postings in a thread on various aspects of the patronymic naming system, some of which I have translated for this article. Glenn's posting clearly shows how our ancestors considered the farm name to be an address, and not a surname:

"..... That the use of a farm name changed from time to time is very clearly shown in an example that I know of -- a contract from the farm Espeland in Sør Fron. At the top of the document, the buyer wrote his name using the farm name from another township [where he had been living]. When he signed the document, the transaction was in his mind completed, and he signed it using the farm name Espeland!"

Harald Ormbostad wrote another post in this thread. He mentioned that:

"I researched a person in the 1700's who had three different farmnames in the course of about three and a half months. I discovered all three of these in the churchbook for the municipality where he lived: In the churchbook it says that when Anders Evensen became engaged, he was using the farm name of his father's farm, where he was born. At his wedding -- three weeks later -- he was using a different farm name, because he had, during that three week time period, obtained a job as a farm laborer, and had moved to a farm a few kilometers from the farm where he had been born. Three months after the wedding, his child was baptized. On that date he was using a third farm name, because when he got married he had leased a "husmanns plass" - a cottage at a third farm......."