Acrobat rescales my image documents. Why?

For a long time, I have created PDFs for presentations of layout for on line viewing or for email attachments, by creating 72 dpi RGB images in Photoshop, with variations of my layout saved as layer comps. I then, use the pre-loaded script in Photoshop(CS2 and3). "Layer comps to PDF". I then open the resulting PDF, and change PDF settings in Acrobat as desired for presentation, such as to make the document open at 100 on in single page mode, centered on screen, printing prohibited, etc, since these options are not availble directly from within Photoshop. I now find that the CS3 PDF document comes out as expected, but when it is opened and re-saved in Acrobat 8 the scale of the images changes. 100 percent view in Acrobat enlarges the images. Since the images are 72dpi, this means they are displayed blurry at 100 percent view in Acrobat. When when the photoshop originals are viewed at 100 percent, side by side with the Acrobat 8 document, the images are not the same size on-screen. I have checked settings Acrobat in 8 carefully, and have tried both saving and exporting from Acrobat 8, but the problem is consistent, and I have never encountered this issue until using Acrobat 8. I and have turned off resampling and compression for images as I always do, I want the images to appear as is in the Photoshop PDF. I even resorted to making my Photoshop documents as 8.5 x 11 at 72dpi, with the images centered, but the issue is persistent no matter what size the originals are. Why is this re-scaling by Acrobat happening? What have I missed?