OK, I looked it up. These are pretty different briefs. The Holder DoJ petition asked the supreme court not to take the case, in a situation where the Federal Circuit had reversed a previous win at trial for Google by saying that "APIs can be copyrighted", something the trial court had assumed not to be the case. So the DoJ was basically taking a position on copyright law here. The supreme court agreed, refused to hear the case, and the net effect was that the Federal Circuit sent it back for another trial on whether or not the court-declared copyright infringement was fair use or not.
The situation now is that Google won at court again. The jury found the API reimplementation was fair use. And the Federal Circuit overturned the trial court again, saying (I'm not making this up, though obviously I'm paraphrasing) "The jury trial we demanded before was invalid because this infringement cannot be fair use as a matter of law, we just forgot to tell you that earlier."
That is, the Federal Circuit is behaving badly here, effectively shopping around for a basis to force a trial court to find for Oracle. The time for principled legal arguments was literally six years ago, this is just partisan hackery.
So obviously Google is appealing to SCOTUS (they took the case this time, for obvious reasons), and the DoJ is jumping in to pick a side in the substance of the actual case, something they really didn't do earlier by requesting that the appelate victory stand.
