Let’s say your state makes you vote in person. You’re handed a ballot and fraudulently vote multiple times, undetected by the busy poll workers. But, your multiple votes are detected later when the roll is scanned. The box containing your ballot is numbered, and workers need to manually find and remove your fraud.

Mail-in ballots are no different. A limited, known number are printed. The system prioritizes in-person ballots in case the mailed one is filled out under pressure. Some states forbid unique ballot tracking, using theoretically non-unique identifiers.