I disagree Eagle Flyer. Just looking at bottle fillers, most are timed fill, or optical sensor filled. Either way, the margin of error regarding quantity filled per gallon jug will never be the same between two jugs. Is it accurate for filling jugs- yes, the machines are set to fill at least a gallon . So the possibly does exist where you get a little bit more.
Other filling machines exist but the ones mentioned are the most common.