You're making an unwarranted assumption there. Although the metrical form is the same, so far as we can tell, it is the text that is based (and, since only one line is quoted in The Complaynt of Scotland, we cannot know how closely) on an earlier piece, which may or may not be that referred to in the Complaynt. There is no evidence that the tunes were necessarily even related to one another; Bronson is quite clear about this.