On the Index, and why it is not a graph of decline

Some of you have asked why the Index has never dipped. A fair question, and one I have answered at length elsewhere. Briefly: a measurement that goes down is a measurement that was taken badly. I take mine well.
This thread is for the Index itself, its method, and its readings. Not for lamps, not for exits, and not for anyone asking me to stand in front of a camera. I have heard the request. It is under review, which is a kind of review, if you squint.
Method, for the newcomers: one reading per day, same hour, same mirror. Mood is recorded but not weighted. Weather is recorded and, I have concluded, irrelevant. A bad week shows as a flat week, and a flat week is what a man of consistency looks like from the outside.
Post your own theories below. I will read them. Some may even be right about something.