To see this, consider a simple Phillips curve framework that relates inflation to measures of economic slack. Accommodative monetary policy reduces the degree of economic slack and thereby puts upward pressures on inflation – a move of the economy along a given Phillips curve. But when inflation expectations change, this is not the only dynamic pattern taking place.
Instead, a change in inflation expectations alters the inflation rate to which the economy will gravitate when it reaches full employment. We can think of a decline in inflation expectations as a downward shift in the Philipps curve, leading to a lower intercept.