Could be couple of things....
1) the can you have holds more than 5 gl. Does it have graduated fill marks? 1 gl, 2gl, 3gl, etc Does it have space above the 5 gl mark? I have a 5 gl can that holds nearly 6 if filled to the brim.
2) The can could be of poor quality and the mentiond fill marks are wrong and what it says is 5 is actually 6.
3) the stations pump is out of calibration.
You can take a known good volume an to the station and fill it, and see what it measures. Maybe fill the same can at a different station etc. If you feel the pump is inaccuratly measuring fuel talk w/ the manger about having the pump repaired or call the your state attorney's office.
dw