The gas pump at your station is calibrated and checked by the state in which it is located.
The computer "thingy" in your car is usually fairly accurate but that's it. I would never believe it to be dead-on-the-money.
It's like your gauges, they're only for general guidance. If you want gnats-ass accuracy, you need professional mechanical gauges, proper wiring, and properly installed senders.
Take in mind that there could be about a gallon of fuel in all the lines. The computer figures it out by how much fuel the injectors have used, not fuel taken from the tank. So that would be where your extra gallon could have gone.
In cold weather, you should NOT top-off the tank. The fuel is cold in your tank and you're adding fuel that is approx. 60 some degrees. Then as you drive the car, the fuel is recirculated AND WARMED. All this adds up to mucho thermal expansion. Don't be surprised if the fuel just starts peeing out the filler neck while running (I've seen it happen).
The other thing that comes to mind here is...When you fuel you don't always get the same amount in the tank unless you fill to the cut off and then top off. Pump rates vary and that will affect the cut off point if it pumps faster it'll cut off slightly sooner. I've found mine to be fairly accurate but it's more of a toy than something to be taken dead serious just like your instant/ average fuel economy. These things are usually fairly accurate but they're subject to a few outside variables.
That's not uncommon among OEM gauges. The OEM has this thing about making gauges read what they want them to. Matter of fact, many OEM gauges are electronically damped. In other words, they make them read certain ways regardless of what the TRUE reading should be. Just because it makes the customer happier and less likely to take it in for repair.
You remember when the GPz900 came out? Maybe Jeff does. Anyway, not long after the bike came out, a lot of customers complained about the bike running hot. The bike came with a wimpy little temp gauge and it usually read toward the top of the gauge. Kawasaki issued a service bulletin on the issue and when we got the repair kit in, you know what it was? How 'bout a resistor that we were supposed to solder in-line with the gauge to make it read a little lower!
Another case in point, several OEMs electronically dampen their gauges in order to have them read in a more confidence-inspiring area of the gauge. It's true dude, I s%#t you not.
That's why I ALWAYS tell people to take what their OEM gauges say with a grain of salt.