It is important to remember that "surnames" were not required in Norway until 1923 (Sweden and Demark make their laws less than 100 years prior). So you will often see various values at the end of a person's name depending on when the name was taken, where they lived, or what they were reporting to.
As was said before names like Olsen, Svensen, Jocobsen (sometime with two "ss" or and "o" or just a dot) are patronymic names that may or may not have become surnames after 1922/23. City people had surnames before the law because of European/Danish influences, while rural names followed the patronymic names (they were not surnames like ours today) very often taking the farm they owned or were born on after 1922/23 or after migration to another country. I have examples of brothers in Norway were one used the patronymic name, and the other used the farm they were born on.
I do not record surnames in my database (my software does not require a surname) unless they are/were used as a surname and passed down the generations, or after emigration to the USA or other countries with true surnames, or in documents after 1922 where the name is official. Some people change their name after 1922 because their were too many people in the town with the same name.