Photoshop :: Document Sizing

I am a digital scrapbook designer and I, and several fellow designers, have just started experiencing some odd behavior in CS.  It is occuring on both Mac and PC platforms and in versions of CS3 through 5.
 
We create 12 x 12 in (3600 x 3600 px) 300 dpi documents used by our customers as "backgrounds" for digital scrapbooking kits. We save the documents as jpg's Basline Standard, anywhere from 8-12. Sometimes, if we open a saved jpg and place (drag) it onto a new 12 x 12 document, we notice that the saved file is just a bit and sometimes as much as an 8th of an inch smaller than the new document, even though they are both 12 x 12, 300 dpi. Additionally sometimes when we drag the saved files from Finder or Windows Explorer to a new document while holding the shfit key to center placement, we're still off by that same 8th of an inch.  Sometimes, if we align the horizontonal and vertical, the same jpg will be perfectly aligned and no longer too small for the 12 x 12 document and sometimes you can very clearly see the edges of the layer beneath.
 
This issue is driving us nuts because it just started happening recently and our customers, who very often have their finished scrapbook pages printed, have also started noticing.  I don't have a clue where start trouble-shooting this.