When I ordered the Aluminum, part of the reason why I ordered it was because it looked good, but I'll be honest-- part of it was because it would ship fastest according to the Google Store. Frost would have shipped a week or two later in 128GB.
So my uneducated guess is that they thought the Aluminum 128s would be done before the Frosts, but they weren't for whatever reasons, and just decided to ship them all out at once.
And I'm glad they're giving us real rebates, and not Play Store credits. That was a classy move on their part. They could have just said, "Too bad, that's the tech biz," but they didn't. It's hard to use Play Store credits to buy beer, anyway.