Why is the backing too short?

Have you ever wondered why, despite careful measuring, the backing comes up short when you get to the end of the quilting? 

Well here's the reason.  Let me show you by doing a picture demonstration.  First find a magazine.  Any will do.  The front of the magazine represents your quilt top. The back of the magazine represents the quilt backing.  The inside pages represent the batting.  Now roll the magazine up starting at the spine and rolling with the front page inside.  This is just like how the quilt, batting, and backing would roll up on the pickup roller. 

See what happened?  The backing comes up short.  Now open the magazine and insert a second magazine inside.  Roll it up as before. 

See what happened?  The front and back pages are even farther apart.  This is what happens with a thicker batting in the quilt sandwich.  When you plan to use a thicker batting, or a very puffy one, you need to compensate for this by adding a few extra inches to the backing. 

It's the process of rolling the quilt sandwich, on the pickup roller, that takes up so much backing.  It doesn't seem logical.  It would seem that the quilt would have a bow in it after quilting but that doesn't happen.  That part I can't explain.