tzor wrote: If you want to challange a specific voter you can and you can specifically eliminate their vote after the fact. You can't do that with election day ballots since the ballots are not attached to the specific voter in any way.
erm... not totally true, tzor
I've been one of the representatives of a party, a volunteer put there in case of any voter being challenged, and in fact, had to work with the registrar because a voter was challenged on the spot. Actually, more than one voter was challenged and of those, all were eligible to vote, but three of them had been sent to a wrong precinct (a snafu in that area because one of the buildings that had been the precinct was undergoing restoration).
Point is, those in-person votes can be challenged and when they are, if they are not resolved right on the spot, they can be put in a special book, their ballot separated for later review, which is what a "provisional" vote is.
The problem with a 'provisional vote' is that those votes will not be counted UNLESS a vote is really close. If there are 1 million provisional votes in an area but the election is separated by 700,000 votes, the provisional votes may or may not be counted, depending on how much a percentage of the population that 700,000 votes represented. It doesn't matter by law if the number of provisional votes exceeds the separation of the candidates, what matters is if the separation exceeds a certain population percentage. (The end result is, provisional votes are almost never counted.)
Votes in-person can also be challenged later, rather than on the spot. This was the case with the infamous "who was really elected?" ballots in Florida, where some of the ballots could appear to have both or no candidates selected because the little paper divet insert that was supposed to come off when that candidate was punched, didn't come off. Per Florida laws, those people should have had the chance to vote again, but the Supreme Court overruled it because of time and money involved to allow it, so Bush got in when maybe he didn't really win that election.
Additionally, while your case is that the voter could be questioned more if it's a mail-in ballot, I dispute that argument on the basis of, it's a heckuva lot easier to see if a man who'se been registered to vote for 60 years really does look like a close-to-70-yr-old-male, in person, than to see if maybe that person's granddaughter or neighbor got hold of and filled out the mail-in ballot with their choices instead.
That vote would only be questioned if it turned out that a significant portion of mail-in votes were suspected to be fraudulent.