QTR-Quadtone RIP

Has anyone had any observation about a very small change in linear
scale with QTR?

My workflow is to do a layout in InDesign, export to PDF, open the PDF
in Photoshop, save to TIF and print. Windows XP. Lastest QTR (2.5.1.0).

Here's the problem. I've made an exact size layout (10.5x8" landscape
orientation) with crop marks in InDesign. The exported PDF file
measures exactly 10.5x8" to the crop marks. Importing the PDF back
into Photoshop, the crop marks still measure exactly 10.5x8". When I
print using QTR (2.5.1.0) at 100% (to my Epson 4000) the crop marks
measure exactly 8" in the paper path direction, but are 0.5mm too wide
in the print head direction. Any ideas why there is a gain of  half a
millimeter in width? This is causing me fits trimming the prints to
fit my die-cut folio covers to perfect centers. I'm open to suggestions.
Brooks