Tank Half Full

Lately I have noticed a trend by rental car agencies to provide a car with half a tank of gas, instead of a full tank.  One can only assume that this is an intentional act to take advantage of the consumer.  Good luck refilling the tank to the exact same level at the end of your travels.  It is much easier to fill a tank completely (starting with a full tank) than to estimate how many gallons of fuel you need to pump to get it to half a tank.  If you fill it with too little fuel, they will over-charge you for the difference.  I'm not sure what they do if you go over half a tank.  I doubt they siphon out the gas to bring it back down to half a tank.  I also doubt they give it to the next person with 5/8 of a tank.  Perhaps they fill it to 3/4 tank for the next person. 

What really has me curious, is how they can fill it to exactly half a tank?