Congratulations! Remarkable find!
Could the change in mileage consumption be due to ethanol in the fuel? I had a similar experience driving north from Miami (10% ethanol for sure) up where the fuel seemed ethanol-free and mileage/gallon increased.
The exact mileage upon purchase was 7,949 miles and upon arrival home was 10,222. This mileage, when I purchased, was verified by the Lexus dealer where it was serviced and by the emission testing compliance requirements of Illinois as well as a Car Fax and an eBay history check. Upon driving it home I now believe the fuel mileage increase was twofold. One was, as you said, the change to ethanol-free fuel and two being a more complete break-in of the engine and drivetrain.