As the other answers say, ANA is happy with the 787 surpassing the 20% fuel savings. But that does not mean for any given trip a 787-8 burns 20% less fuel than a 767-300ER. Which makes your point about Air India valid, here's why:
It's not a fair comparison. The ANA 767-300ER on transpacific routes is not in its natural habitat, i.e., it can fly the mission, but it can't carry a full passenger load alongside the lucrative freight.
An academic source mentions the SFC (in cruise) of the GEnx-1B64 being 1.52537x10-5 kg/Ns. Or 0.53852 pounds of fuel per pound-force thrust per hour. Compared to 0.576 for the CF6-80C2B2 on the 767-300ER. In other words, the new engines are 7% more efficient.
Note: The thrust of both engines is quite close at sea-level (3.7% more for the 787-8).
(Own work via boeing.com) Overlayed scaled drawings.
The 787 is marketed as 50% composites, which is lighter. So let's compare the weight to a plane of roughly the same dimensions and passenger capacity: the 767-400ER (not the smaller -300ER).
The Operating Empty Weights are 120 and 104 tonnes. The 787-8 is 16 tonnes heavier. (Strange, right?)
Two things stand out from the drawing above:
The 787 is wider and a bit shorter, which allows wider seats* but neutralizes the composites advantage.
Since the construction materials are lighter, the wings can be built bigger for the same weight. The increased span improves the lift-to-drag ratio, which permits higher speed for the same thrust, and/or the ability to carry heavier fuel loads for increased range.
* Not all airlines will install them of course, but Japan Airlines does for example.
The extra efficiency for the 787-8 comes not only from the new engines, but by also being able to fly faster. If you spend less time in the air, the 7% fuel efficiency is increased further.
For the GEnx, GE advertises on their website:
Up to 15% improved fuel efficiency compared to GE’s CF6 engine.
I ran some numbers based on a hypothetical 200-passenger flight to get ballpark figures. The first row is the new engine, and the following rows are the new engine on the new plane.
Scenario Pax Seat mileage
New engine 0 - 7%
+9.5% speed 0 -15%
+9.5% speed +13 -20%
+6.3% speed +19 -20%
The +9.5% speed example is going from Mach 0.79 to 0.865, this achieves the advertised GE value.
If Air India is flying shorter or high-density routes, then the full 20% won't be realized. (Air India are very much aware of this, each airline is sold the right projections based on their operations.)
From Wikipedia for the transatlantic routes we have a fair [and well-sourced] apples to apples comparison (I added the rightmost column).
Model Pax Distance Fuel/Seat Per seat saving
787-8 291 6,300 km 2.26 7 %
767-400ER 304 6,047 km 2.43 5.08%
767-300ER 269 5,600 km 2.56 -
Note that the -300ER burns more fuel per passenger while carrying fewer passengers a shorter distance at a slower speed compared to the 787.
To adapt the -300ER for the transpacific, the plane will not be fully loaded, further increasing the fuel per passenger. This allows ANA better efficiency figures when they compare it to the Dreamliner.