Looking at the past few days of the IAH-SYD flight, it actually does fly a little bit north of the GC route - flying near to that "point" of ETOPS-180 restricted area that LAX-AKL almost crosses. It's obviously not necessary it take this route as the 787 is rated for ETOPS-330, and I presume that United would've certified its planes and crew with at least ETOPS-240 so it could fly the GC route or slightly south. That being said, it's possible I guess that United did not pursue higher than ETOPS-180 certification and that is the reason for the northerly flight path of that flight. Or maybe they're taking more caution than is necessary.