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. Frederick I of Denmark:  As King of Norway, Frederick is most remarkable in never having visited the country and was never being crowned King of Norway.
never having visited the country and was never being crowned King of Norway.