I think the computer seriously buffers the gas meter reading when the gas level is going up. It probably instantly calculates a low reading and displays it - so as to err on the side of saftey (i.e. you not running out of gas)...

BUT when it detects MORE fuel than the current reading, it waits a long time and gets multiple samples to ensure that this is correct (so it wouldn't behave like the example of AZCivc of going around a bend and showing a ton of fuel)

Here's how I came to that conclusion... I also experince the "almost instantaneous" drop in fuel bars every now and then (I live and work in a very hilly region). But today, when I filled the tank (12 gallons) and left the car on when it was filling (the engine was off ) it took me about 8 miles of driving before the gauge finally read "full".

The whole time it was going up bar-by-bar; VERY slowly.