It's unlikely your actual, physical monitor resolution is 72 ppi.
Courtesy of the late John Slate:
John Slate - 3:08pm Jul 15, 05 PST (#7 of 10)
If you want to get exact about displaying print size:
Open a new 500 pixel x 500 pixel file in Photoshop, and set it to 100% (view actual pixels) if it does not come up that way.
Get out a ruler and measure the image on screen in inches.
Divide 500 by the measurement.
This is the resolution of your screen, write it down somewhere. Call it your actual monitor resolution factor.
That is the figure you need to input in the "Screen Resolution" field in that Preferences dialog box.