This has been a very interesting thread..Thanks to everyone. One thought occurred to me is it possible that the step-wedge is coated or incorporates some type of UV filter to protect the steps from fading? This would make some sense, because over time would UV light not cause a change in the step-wedge?

Since I do not own a step wedge (on the list of things I need to get) and not a practitioner of BTZ, I could be off base...just wondering though. It would explain the results reported I think.