I think the logical conclusion of copyrighted prompts would include not just the input prompt but the version number of the program, any settings, the seed, etc. Basically everything you would need to copy that exact image, because all of that together would produce an exact copy.
The reason the image is different using the same text prompt is because it randomly generates a seed for each time it runs. Presumably the copyright would include all the settings, including the seed. All of that kept the same will produce the same image, every time.