Where are you getting that from? Your screenshot shows that one will burn through it's 50,000 litre reserves in 520 days and the other will do it in 833 days. Am I missing something?
Edit: Unless you mean you've done a practical test of one day. In which case, it might be an idea to test how far they can actually go. If they both have the stated range, then they're using fuel correctly. If not then I'm sure it's a bug.
Well the "maintenance" tab seems to indicate that both ships are 48 litres short of their maximum, presumably having both run one day.
The Ark Royal however should use 96 litres a day at top speed and 37 litres at its current speed of 1722 km/s.
The Agincourt should use 60 litres a day at top speed and 54 litres at its current speed of 2500 km/s.
So
if both ships ran at their current speeds for one day, starting with full fuel tanks, than they should not both be 48 litres short. Then again all sorts of funny things seem to be going on with the crew and casualties, so I would not know what "should" happen to fuel requirements in that case.