Actually, this bug has persisted since Snow Leo. It seems that you can actually never print at actual size, i.e., 100%.
The only work-around is to set 100 in scale after all settings and then press Enter. Clicking Print or moving out of the scale field resets the value. This bug has survived an amazing 3 releases of the OS and all related updates thus far.
After searching around on the net and checking out various options, the one that actually works is changing the defaults in a terminal as follows:
defaults write com.apple.Preview PVImagePrintingScaleMode 0 defaults write com.apple.Preview PVImagePrintingAutoRotate 0
Ideally, you would set both these defaults to ensure “normal” behaviour.
Nitin Chandra's Blog 