Why do My Country tis of Thee and God Save the Queen have same melody?

"My Country, 'Tis of Thee" and "God Save the Queen" do not have the same melody. The melody for "My Country, 'Tis of Thee" is derived from the British tune "God Save the King," which is also the source for "God Save the Queen." However, the two melodies are not identical. "God Save the Queen" and "God Bless America" are the songs that share the melody.