On the bows I've checked, 26.25" to the throat of the grip generally equates to 28" to the back of the shelf, or too close to worry about it.
AMO does specify 26.25" from the throat of the grip for a 28" draw weight specification.
www.texasarchery.org/Documents/AMO/AMOStandards.pdf Some refer to measureing to the throat of the grip as "true draw", which makes it confusing and will make the numbers seem much better than they actually are if you don't know any better. I.E., 28" "true draw" is actually almost 30" AMO (29.75").
I also agree that the scales used can be a big cause of variance. I've heard of some bowyers, and seen several shops, using regular old hanging scales (clearly marked "not legal for trade").