A different manufacturing method or a different material was definitely used.
Look at the iPhone 6 (Plus) on the right with the "milling marks" left behind compared to the left where it has a smooth, continuous flow process
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The majority of the "significant" increase in stiffness is from the changes in the manufacturing methods. The new 7000 series aluminum is not a big deal, IMO.
The Ribbed Pattern on the back is very rigid.
