A performer once remarked that performing a show every night is like taking a boat out on a lake: it’s the same boat and the same lake, but the wind will take it in a different path every time.
It’s the same show, with the same group of actors, but every night there will be slight variations. As shows progress over long periods of time, it’s hard to notice the slight variations that can change a scene: a line is dropped, the staging might shift, the vocal harmonies might adjust. All of the variables can and will happen.
The maintenance of a show is important; once a show is set, the lights always hit where the staging is set. The music is set with the harmonies. Every line of dialogue is important to cue the music the lights or sound. Consistency keeps the show at a level of excellence, and even within the consistency there is room to create a unique path for every performance.