Here's a topic that's right up my alley. Truth be told, of all the info gauges, the gallons used is probably the most accurate, but it likely won't line up with the pump unless it's Spring or Fall. Gallons used is calculated from the fuel injectors pulse width. What can throw it off is the same thing that will throw off the fuel pump at the filling station - the Specific Gravity of the gas. For you non-scientific types, Specific Gravity (SG) is how dense a liquid is. Water is 1.00 - about 8 pounds per gallon. Gasoline is lighter than water for the same volume, about 6 pounds per gallon, or about 0.75.
The problem is that the SG of Gas isn't constant. It varies between 0.72 and 0.78, based on temperature, vapor pressure, additives and so on. Since it's really expensive to check the SG of gas in a vehicle, the calibration point is based on the average of 0.75. As it gets colder out, the gas will get denser - around 0.78, and as it gets hotter, gas gets thinner - around 0.72.
To put that in English, gas shrinks when it's cold, and expands when it's hot. During winter in the northern states, your gas will shrink when it gets inside your tank and cools down, making you seem to put more gas in the vehicle than what the info center says you used. The exact opposite holds true during the summer - you may put in less gas than what the info center says you used.
Fuel station pumps have the same problem with the changing SG. Since the underground tank temperature is pretty constant around 50 F, there isn't much variation once it gets into their tanks. But if it was 30 F when they got their fuel, they likely got more fuel from the tanker than they would get if the tanker came when it was 90 F outside. Station pumps also aren't perfect in their calibration - we have a calibrated fuel can when we go testing that holds 5 gallons exactly. We've seen values at the pump from 4.65 gallons to 5.20 gallons when we have 5.00 gallons in our can. It's likely due to temperature and vapor pressure of the fuel, not anything intentional, but thestation pumps aren't perfect.
There's one other thing that can throw off the info center - how often you stop and start the engine. During the crank, the engine dumps fuel that the injectors and computer can't count accurately If you crank for a long time, and make lots of short trips, you have more time that the computer can't calculate, and consequently it looks like you used less fuel than you actually did.
So where does this leave us? With a bunch of gauges on both vehicle and station pumps that aren't anywhere near as accurate as we'd like them to be because of how the gas changes as the weather changes. If you want to know the truth I always run based on the gauge and the miles driven. I purposely run out of fuel on my second or third tank of gas with a jerry can in the trunk, so I can get a good idea of how far I can drive on a full tank.
PS- don't believe you really have exactly 31 or 37 gallons in your fuel tank either - they are likely about +/- 1.0 gallons from being accurate, but that's another story...
But realize that your mileage is probably lower than what the DIC reads out. 
