Check your owner's manual. For 2005 the tank capacity is listed as follows:

Normally

Aspirated Supercharged

Tank capacity22.4 US gallons 22.3 US gallons

85 litres 84.5 litres

Fill capacity 20.9 US gallons 20.3 US gallons

79 litres77 litres

Reserve capacity 1.5 US gallons 2.0 US gallons

5.8 litres7.5 litres

My 2004 has a smaller tank 18.7 gallons.

 

The neck probably held .5 gal for some reason the sensor on the fill nozzle was reading your tank fill wrong. My wifes Mercedes would not shut the nozzle off and it spilled over everytime. Still does occasionally to this date. We now just listen for the noise of the air cavatating out of the neck and stop filling and it's been working for us.

 

well when i first got my car i had two trips to the dealer where htey did a bunch of little things such as my fuel sending units. I bought my car in Texas and drove it back here to Florida. On the way back on my first tank of gas ever the computer said i had 0 miles to empty yet the gauge read 1/4 tank left, so i just ran it down until the light came on and then totally filled it up and it worked great after that, even before they replaced the sending units. so i think they are being truthful with you and as with any part sometimes they are partially or totally faulty from the factory. As long as it seems to be working now I wouldnt worry about it. the difference between your old fill and new was perhaps it wasnt seperating the reserve and the regular parts of the cell correctly. I think this was the case with my car also.