From my understanding, there are a few possible reasons that may be the case.
The position of the some fuel sensors and the shape of some gas tanks are not well-suited for accuracy. Fuel cells for cars designed for racing are usually cubes or rectangles in order to counteract that particular issue (among other reasons).
Additionally, fuel consumption is based on the amount of work the engine has to do in order to move the car. The greater the force of gravity on the car (that is, the heavier the car is due to a full tank of gas), the more work the engine has to do to move the car. Due to this, fuel efficiency is almost always lower when the gas tank is full.*
*If the car is rear-wheel drive and the gas tank is positioned over the rear axle, extra weight in the back will help maintain consistent contact between the tires and the ground. In racing conditions, added weight in the rear of a rear wheel drive car is usually beneficial for both speed and efficiency. Alternatively, if the fuel pump is shoddy, the increased pressure in the gas tank may increase fuel efficiency and therefore compensate for the increased weight.