In the context of a low-inflation environment, firms may have concluded that they can leave their prices fixed for longer periods at little cost, causing inflation to be less responsive to shocks, particularly if they are transitory. In this way, monetary policy may have affected the slope of the Phillips curve without having affected the manner in which expectations are formed.
Also, from the mid-1980s through the first years of this decade, energy price movements were smaller and more transitory. We see this in futures markets, in which oil prices in far-dated futures contracts moved much less than spot prices over this period, suggesting that people expected a quick reversal in any rise in oil prices.