Why was Frederick I only called "King of Denmark" while Christian III was named "King of Denmark and Norway"?
Frederick I of Denmark Frederick I (7 October 1471 – 10 April 1533) was the King of Denmark and Norway. His name is also spelled "Friedrich" in German, "Frederik" in Danish and Norwegian and "Fredrik" in Swedish. He was the penultimate Roman Catholic monarch to reign over Denmark, when subsequent monarchs embraced Lutheranism after the Protestant Reformation. As King of Norway, Frederick is most remarkable in never having visited the country and was never being crowned King of Norway. Therefore he was styled "King of Denmark, the Vends and the Goths, elected King of 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.