lots of reasons - relatively less fuel-efficient aircraft, higher number of premium seats which boosts the 'per passenger' figure; cargo loadings on US flights probably result in a higher average take-off weight for BA than most other airlines (take-off and climb being by far the most fuel-intensive phases of flight); also I expect some small contribution from BA being primarily out of LHR where taxi and waiting times tend to be longer - this causes not insignificant fuel burn before you even leave the ground - fairly common to have a 744 taxiing for 45-60 minutes from the gate to take-off at peak times which must make a difference across the dozens of daily flights