[phenixbb] Why the correlation coefficient of the electron density with the model is never 1?

The links below show a plot of the results of phenix.real_space_correlation* for a nice lisozyme crystal and also the coot view. https://drive.google.com/open?id=0B9tqXyDsq9YIdkRhcVFXU2dEdnhINXR6Y3hza2Y5NG... https://drive.google.com/open?id=0B9tqXyDsq9YIREd1OFZhNGZtRnN4aldtVU1mdk84NE... As you can see the electron density fits very well the model. Why not a single residue gets a perfect correlation coefficient? (i.e. 1) Just curious. Thanks * phenix.real_space_correlation pdbfile mtzfile --detail="residue" > outpu