Why does my 2003 Honda Accord fuel actual tank capacity differ from the official specs?

I have filled up at the gas station multiple times in my 2003 Honda Accord LX. I have noticed that each time I fill up it is within the 11-13 gallon range. So according to the gas stations, my gas tank is about 13 gallons. I looked online and I have found the spec sheet and information has stated that the fuel capacity is 17.1 gallons.

Why does my actual fuel capacity differ from the 'official' capacity?

Thanks!