I have also had this problem at Hertz in Sweden (drop off at Stockholm, express drop off out of opening hours). They overcharged me about 200 sek (just over $30), I went to the pick up station (In another city) to ask why this was charged, they said that it was because the tank was not full. The drop off location was at a petrol station, so we willed it up and drove a couple of meters to drop off point, but as it was a friend that filled the car with petrol, I was not sure that it was 100% full, so I just paid. (Later on he did tell me that he was sure that it was full)

So it seems that this is not limited to the USA...