you asked for it so I will correct you !
the max payload of the A380 is NOT 66t
the comparison is a false one as comparing a 777-300 at 360 seats and an A380 at 489 seats is not comparing apples with apples
the A380 is carrying many more 'premium' passengers than the 777-300. If you were to configure the 2 a/c with similar cabins the A380 would be carrying around 600 Pax
the A380 is carrying lots of additional weight compared to the 777-300 - showers, bars, etc. This all costs fuel burn, but the airline clearly thinks it's worth it. The A380 is probably carrying additional reserve fuel so early in it's operational life;