Just confirmed what alot of folks already know

Hand calculating has a lot of variable; same pump, (each pump, ea station can have different calibrations, slight yes, but still different), fill it up exactly each time, time of day, same temp, elevation (as IdaD commented), rounding, speedometer accuracy. The vehicle computer; distance / fuel flow rate. The major variable is tire wear (speedometer accuracy).

IMO - Vehicle computer is more accurate but it is fun to compare, I do.