When you try to calculate mathematically from the total fuel and the range, you’re assuming that fuel burns are linear. I don’t fly big aircraft, but the bizjet I fly carries just over 19,000 pounds of fuel (6.7 pounds per gal, not 6)
A recent trip was flown at low altitude because of strong headwinds west bound. It took 3.2 hours and we burned 11,100 pounds or 1657 gallons.
That works out to 3469 pound (518 gallons) per hour.
The return trip - same mileage was flown at normal altitudes. It took 3 hours and we burned 7270 pounds or 1085 gallons.
This is 2423 pounds or 362 gallons per hour. Big difference.
Another consideration is ATC … sometimes you can’t get as high as you’d like, particularly over the North Atlantic.
A final thought…generally speaking you burn 10% of what you carry, so if you take 1,000 extra gallons of fuel you will burn 100 gallons to carry it.