If you record the fuel added to aircraft, the system has been showing the average fuel consumption based on the previous 30 days flights.
But we've found over the winter this just isn't accurate enough because the flight time for some aircraft has dropped to single digits per month. That means we're dividing by a small number, which leads to a big variation in the fuel consumption figure.
So we're now running a more thorough calculation, based on the last 100 hours engine run time instead of the last 30 days of flights.
That means we're summing the total fuel added (again, assuming you're recording this when you book out) and dividing by the actual engine run time. For aircraft where there isn't a history of the last 100 hours yet, the calculation still works, and gets more accurate as more statistics are available.
Of course this does rely on having accurate figures for the fuel added, but a few litres won't cause a noticeable difference.