InDesign Measurements don't add up

Laying out a book cover, (which I've done dozens, and dozens of times) and the dimensions and bleed that I set in the Document Setup don't match the sizes of the page elements. A full bleed image that matches the dimensions set up with document setup/margins/bleed doesn't line up. The cover is 5.25" x 8" with a .372" spine. So, the overall trim size is 10.872" w  x 8"h, which is what I set up in Document Setup. There's a .125" bleed all around, so size with bleed is 11.122" x 8.25".
With me so far? So, if I make 3 picture boxes... one for the back cover, one for the spine, and one for the front cover, with the measurements: 5.375" x 8.25" (front & back covers) and .372" x 8.25" (spine) and I snap all the edges together along the spine, they should all line up with the bleed lines, right? Nope. Off by a little all around, which is a problem when sending PDFs to Amazon P.O.D., who doesn't like a 2-thousandths descrepency at the bleed line. Two thousandths at the bleed line normally wouldn't be a problem, but at Amazon, the people rejecting this don't know anything outside that ... the numbers they expect don't match the numbers they see. 
For the record, I've been working professionally with InDesign for a long, loooooong time, and QuarkXpress for a decade or so before that, and Adobe PageMaker before that . . . so I'm not exactly new to this. 
So, why don't things line up the way they should? 
See attachment.
I zoomed in so you can see the descrepency, and I enhanced the bleed lines to make them easier to see.