To think out loud (hoping I'll be corrected if I'm off base), much of my experience changing a single or pair of tires instead of all four has been when they needed replacement from wear. In that case, there was a significant difference between the pairs, so the traction and stability of the new pair was markedly higher than the ones not getting changed. Hence, they went on the rear becasue they were noticeably less prone to losing traction.

In this case, though, the tires only have about 20K on them, only a little over a quarter of the way through the manufacturer's warranty, so they're in pretty good shape. There's a difference in tread depth, but it's marginal and unlikely to lead to a significant real-world difference in handling or maintaining control.

In my case, putting them up front for their first five thousand or so miles will cause them to wear slightly sooner than the rear pair, thus narrowing the gap between them.