If my car is on empty and I pump only 3-4 gallons, the gauge doesn't move at all. The owner's manual does reference this fact, so it's not a defect.
Does anyone know why it works that way? I find the gauge to be absolutely accurate from full to empty. Is it to keep us from driving around with very little gas in the tank?

 

as far as I know porsche tank designed that way thay the gauge can not go below 50% of the tank so it calculates the ammount of fuel after 50% from the injection consumption. so in order to the sensor to reset and see fuel, is to pump above 50% and then you will see the gauge readings again

 

996/997 turbo's tank is built around FWD driveshaft so that the tank gauge can only measure down to 27 liters, rest is calculated from fuel flow by the Motronic. If you fill less than 27l of fuel the tank gauge has no contact to fuel and can't initiate a new countdown cycle.

Ie. fill it up, man!