Bunads differ according to region, it hasn't got anything to do with being married or not. This particular headpiece, called hårvippe belongs with the bunad from the region of Telemark. For Sverre Magnus' confirmation Queen Sonja, Ingrid Alexandra, Märtha Louise and Leah wore Telemark bunads. It's the region of origin of Sonja's mother, she usually wears that region's bunad, as does Märtha Louise, and Sonja and Harald gifted Ingrid Alexandra and Leah with Telemark bunads for their confirmations as well.
Märtha Louise usually wears the corresponding headpiece, it was an exception that she did not yesterday. At Ingrid Alexandra's confirmation, Marius' girlfriend also wore a Telemark bunad, thus the headpiece. Yesterday, she wore a bunad that I think is from Vestfold (which is apparently her region of origin). Mette-Marit wore a Kristiansand bunad yesterday (as she usually does) because that's where she is from and the headpiece doesn't go with that bunad.