Why was Frederick I only called "King of Denmark" while Christian III was named "King of Denmark and Norway"?
Election of Christian III The election of Christian III as king of Denmark and Norway on 4 July 1534 was a landmark event for all of Denmark and Norway. It took place in the church in the town of Rye, eastern Jutland, where the Jutlandic nobility elected Prince Christian, son of King Frederick I and Duke of Schleswig and Holsten, as king. This brought about the Count's Feud ("Grevens Fejde") and later also led to the implementation of the Protestant Reformation in Denmark and Norway.
Margaret I of Denmark Margaret I (Danish: "Margrete Valdemarsdatter" , Norwegian: "Margrete Valdemarsdatter" , Swedish: "Margareta Valdemarsdotter" , Icelandic: "Margrét Valdimarsdóttir" ; 1353 – 28 October 1412), was queen consort of Norway 1363-1380 and Sweden 1363-1364, and later ruler in her own right of Denmark, Norway and Sweden, from which later period there are ambiguities regarding her specific titles. She was the founder of the Kalmar Union, which spanned Scandinavia for over a century. Margaret was known as a wise, energetic and capable leader, earning the nickname "Semiramis of the North", or "the Lady King". Though the latter was a derisive nickname invented by her rival Albert of Mecklenburg, it became widely used in recognition of her capabilities.