Also remember, as others have suggested, the fuel tank is 24.6...so there is a 99% probability that you had approx a bit more than 1/2 gallon of fuel left (0.7 gallons if we do the math). But depending on barometric pressure and other variables like how much fuel continued to flow into the tank just before the auto shut off etc...one can overfill the tank and fuel can rise up into the fill line...so more than 24.6 gallons of fuel can be put into the tank.
The OBC may not have the same avg mpg as your personal figures...again there are variables in play that we may not be accounting for. But there is a way to adjust the calculation if it is way off. If you've unlocked your OBC, you can adjust the calculation with
TEST 20. You may not get your personal calculation & the OBC to be "exact"...but you can get them to be within tolerable proximity.
My suggestion is to track your mileage & gallons at the next few tank refills then average those numbers so that you have ONE "actual mileage" variable that you can plug into the correction factor equation...the correction factor equation (VK=correction factor) is:
(Actual MPG / Displayed MPG) x 1000 = VK