It's based on the resolution of your monitor.
For example, if your resolution of your monitor is 96ppi - then it will be
72/96*(100) = 75 pixels being shown at 100%
The zoom scale is changed with CS6. In previous versions 100% was the same as Photoshop where the pixel to monitor res is 1:1. In CS6 100% is the actual print size.
Double-click the Magnifier tool to get the old 1:1 ratio
Great, this works just fine!
Thanks for the tip, Rob!