Its not low actually, last time you didn't fill the tank full, who knows how much was left in the tank and then you filled again so the ratio was imbalanced. Just to have the accurate figure always fill the tank to full when the nozzle ticks. Also the average fluctuate according to your drive, maybe you would have driven the car very near distance due to which small gears were used. Also the petrol station from where you filled up the tank matters the most too!