Kevin
AFAIK this is not a glitch, the fuel allocation is a conservative estimate, so if you leave it alone you will guarantee to finish even if you run full PL5 for the entire race.
Most likely the range of default fuel allocations were a result of teams having differing FE development.
The skill comes in working out how much fuel you'll need and reducing the pre-set fuel load to something more reasonable. If you hit the sweet spot (between 0.1 and 1.0 litres remaining at the end of the race) your car will be fast because it's not weighed down for the entire race with excess fuel. If you get too greedy you will run out and not finish.
Many thanks!