In the Netherlands, I know that for instance in the Netherlands you can give your children doopnamen
"baptism names" (official FN + MN(s)) and a "calling name" roepnaam
(I think that would roughly be nn is English).
Having worked in the itnernational office of my school, I know my school registers students with "official names" and "calling name", so it's pretty easy to see who goes by their MN and who goes by their FN or something else completely.
I believe you can register this when the baby is born. My brother is for example Constantijn
" and I was supposed to be Maria Johanna Catharina
", but something got mixed up with the baptism and calling name and I ended up with Marjolijn Maria Johanna Catharina
on my birth certificate. (my parents are still debating wether this was my dad's or the clerk's fault :P)
Not sure if it's the same thing in Sweden, but at least it would explain it.