It probably still was a fuel charge and the fuel entry was incorrect. There are two ways that they use to calculate fuel and both are supposed to produce about the same result but if the options aren't applied when they are supposed to then the difference could be substantial.

Hertz measures fuel by 1/8's of a tank. When a car leaves on full, it leaves with an "8" and when it returns, if on full, it will also be an "8". Now, if he returned on 3/4's then they would put in a "6" but they may have put in a lower number of put in a "N". "N" means "No" which means that the customer did not purchase any fuel during their rental. This is useful for people who rented for a short period, drove a few miles, and then returned it. The return rep will ask, "Did you replace the fuel that you used?" and if the customer says no then the "N" is entered and fuel charges are applied at a per mile rate equivalent to $7.99 / gallon. Once you have used a tank, though it really begins to screw up the cost and will begin charging for every mile driven from the beginning until the maximum charge is reached.