I am not sure why you have been downvoted. Yes the placebo effect is often just a regression to the mean of a condition that varies. Imagine that you have a disease that goes up or down in serverity (say depression). People enter the trial when most ill and over the course of the trial they trend back to symptomatic mean (i.e. they get better on their own). This is why doctors favourite treatment "let's wait and see" is so effective.