Predictability of Monetary Policy

I have a quite strong economic background, but i have some difficulties with this one. Now, agreeing to rational expectations, if the monetary policy is fully predicted, changes shouldn’t have any effect right? I completely agree with the fact that the central bank has to be credible for its policy to be effective, but i don’t see why they say that non-anticipated moves have less impact. I agree that it is better for central banks to have a stable policy in the long run and to be anticipated by financial markets in order to build credibility, but still i think that unexpected moves should have more impact than fully expected ones. Anyone can help please?