Gas tanks have baffles around the float to slowly let gas in and out, this way the gauge doesn't go crazy every time you take a corner. Some let the gas in and out very slowly, so it takes much longer to reach full.

If it' a real issue, fill up 20 miles or so before you return and top it off at the station at the airport. This way it will show full.

It's not that hertz is rigging the cars, it's how they come from the auto makers.