I was very puzzled about this for months (the longer flight is more profitable, so you would think that would get commercial preference), until I determined that LA had made some service commitments to NZ, so some number of seats are reserved for NZ origin and not sold through to Oz. Making this check much more important than normal (and given the travel patterns of NZlanders outside of FT, much more availability ex-AKL than ex-SYD).