It's interesting to me the language calling it a "metal rim" and "metal band", when Jonathan Ive says it's a solid slab of metal, that the groove is machined in. The iFixit teardown makes it appear that the groove is machined in as well, and the metal you see is all part of a slab.
I contrast Ive's comments and iFixit's teardown with the Keynote presentation, and think someone's simplifying something in the explanations.
From what I've seen, on the right hand side, the groove is effectively 'fake' - designed to create a symmetrical look. One part of the rim is small (3G), and the other is larger, (WiFi) and has the 'fake groove'.
I contrast Ive's comments and iFixit's teardown with the Keynote presentation, and think someone's simplifying something in the explanations.