“Regression to the mean” is the science-y way of saying that things just get better eventually. Here’s a poem about it:
If something varies normally between two far extremes,
It usually swings back naturally to values in between.
And two minutes more of that. Back pain gets a mention. RttM is indeed the major-est of ways that people are fooled into thinking a treatment worked, when in fact they just “swung back naturally the values in between.”
For a much more detailed discussion of this topic, see The Power of Barking: A silly metaphor for a serious point about correlation, causation, and how we decide what treatments work.