Xerox Phaser 7500 Print Sizes

I have a Xerox Phaser 7500 printer.

I have set up a custom paper size (SRA3 - 450mm x 320mm) on the printer itself, as well as on the Windows Preview paper option.

Document size in Photoshop also set up as SRA3 - 450mm x 320mm

When I print an image the actual size that prints is a tiny bit smaller, not a lot, but enough.

For example, a 95mm block will print as 92mm and a 280mm block as 270mm.

I have tried the standard settings, as well as "fitting the image to frame" option in Windows Preview while setting the margin to none.

On both occassions the image will print the same size.

Any ideas?