By the way thinking about and planning an interpretation in advance is as essential as essential as learning and rehearsing the tune and text before you get up and sing it.
Interpreting and identifying with the story and the emotions suggested by the unfolding of that story is the best reason in the world for learning a song and identifying with that interpretation is one sure way of keeping it in your repertoire.
If you are talking about technical variation - singers seldom sing songs the same way twice - they develop a repertoire of skills and draw on those in each performance - like a craftsman selects the appropriate tools for a specific task.
The wider the selection of tools and the better you are able to handle them is what makes for a better singer.
As I have said several times and you have chosen to persistently ignored - resident singers were asked not to repeat the same song over a period of three months - I was there enough to know that this was fairly closely followed, so the likelihood of any of them repeating the same song in the space of two weeks is extremely unlikely, as is the ability of a casual visitor to be able to claim - with any degree of accuracy, that they could tell (over that period) that two performances were identical other than in text and tune.
Jim Carroll