Was it all part of conspiracy to invalidate Obama's presidency:

First the faux birth certificate issue, now this. Ya gotta watch those waskly Republi-cons.
Another possible reason might be that Justice Roberts was reading from Bush's copy of the Constitution:

Of course, those who know Justice Roberts know that he has a fetish against 'split verbs' and "his inner copy editor overrode any instincts toward strict constructionism and unilaterally amended the Constitution by moving the adverb “faithfully” away from the verb."