I can´t really think of any good excuse for the loose loop, but I think the Pochette Metis is indeed one of those bags that are difficult to manufacture, because it´s small and has lots of compartments and therefore needs lots of stitching, glacing and aligning of canvas. The Reverse needs even more of this detailed work, because it has two different canvases.Don't you think it's bad that we have to put up with flaws? I know nothing is a 100 % perfect. Why produce a strap where the second loop is slack. It's not fit for purpose. Not good on a bag that is mega money. It's almost like the design is too complicated with so many issues.
So I can understand that it´s difficult to get a nearly perfect bag. Maybe I was lucky with mine, it´s almost flawless.