Um, that's the entire point of what I wrote.
The 787 is heavy. It's heavy because it carries a lot of fuel. It's less efficient because it has to move so much. As a result, even though it's, per PR-speak, "20% more efficient", that's not taking into account the fact that a lot of that added "efficiency" is simply moving its ponderous bulk around (not just fuel, but more cargo space as well) and does not mean that it's burning 20% less fuel per-seat. On a per-seat basis, it's actually not a heckuva lot better than a 764ER.
On a per seat basis, it actually looks worse than 764ER. As we discussed in another thread, 767-400ER receives no penalty in AWS because it cannot carry as much cargo, and 787 gets no benefit for being able to carry more cargo.
If in the future versions of AWS we do have cargo, 787 will be more competitive. In the meantime, we can either just live with stats as they are, or possibly subtract the extra cargo from the planes and extend their ranges and possibly lower fuel consumption. But that is probably too much work and not a worthwhile exercise...