In Sweden, the "default" name of a person is the name they call you (I'm "Niklas") and the last name is your father's name. You may also take your mother's name or any name that is approved.
I don't know why I don't have my father's last name but I don't since I was about 7 or 8 years when the last part of my name was dropped. I was born H. N. E. Rosencrantz-Larsson and then the Swedish tax auth means that I couldn't have a double last name for some reason so they dropped Larsson although Larsson is my father's name and Rosencrantz is my mother's name.
Now I wonder if there could any more complication that misunderstanding of my father's last name because people assume that my father's last name is Rosencrantz. I'm not sure about my personal preference, but I think it could be less misunderstanding if my name was the name everybody calls me ("Niklas") and my father's last name ("Larsson") or is there a disadvantage for me now to use the form of my name that would "normal" because it is accepted that I am "Rosencrantz" but my father is no Rosencrantz.
I don't think there will be a great misunderstanding, and my personal preference is the name I would have if I was born and named according to the most normal convention we have: The first name they call me ("Niklas") and then the last name is my father's name.
If it doesn't matter and I want to switch, can I claim that I should be approved to switch for free since I'm switching to the "default" and I never was given the chance to have my "default" name?