I have a 2004 Prius, I bought it in Aug '04 and I have just under 20,000 miles. I had it in for the recall a few months ago and haven't had any other problems.

 

I was trying to figure out how much gas each bar stands for.... Hell I fill up when I hit the last bar,

 

After 24 tanks and 12,000 miles I have found a few consistent patterns but primarily each tank gauge varies due to the factors mentioned in other posts above. One gauge measure that has been the most consistent is the mileage I have at half tank--when five bars have been "used".
I am usually at 300 miles plus/minus 50 miles at half tank.

 

I'm inclined to believe that your experience is probably one-off. I'd see if another tank does the same thing.

If you gas up at the same place notice how each pump operates. If a pump filler valve is old or messed up in any way that can have a pretty significant effect on how well it fills up the Prius tank. Some pumps are very sensitive to back pressure, which most conventional tanks might not have much or. However I firmly believe that the bladder has a great deal more back pressure than a conventional tank and as such a sensitive pump will make filling difficult.