If you ever have a "funky" tank and suspect the calculated results to be erroneous, I think the best thing to do is average it in with your next tank. So you take the miles and gallons from this short, possibly wrong tank and add them to the miles and gallons of your next, presumably longer and more accurate tank. Your lifetime average will work out exactly the same but you won't have the spike on the graph.
Why so high well think about it - the gas came out of the ground and got warmed up in your tank and expanded. This prevents you from putting more gas in the tank. If you drove the entire tank you would get the same mileage because the total energy is the same it just tankes up more room than usual. So maybe you expanded a half a gallon or more in the total tank but that keeps you from putting more in. If you filled the tank and it expanded a half gallon and you drove it and burned half a gallon it would still be a full tank.