Hypothetically speaking, they might have added the same-day registrants to the “registered voters” column, thereby increasing it to exceed the number of votes cast, i.e. pushing the number back below 100% - without touching a single ballot.
Wisconsin permits it, so people using the number of "registered voters" are using information that is inherently out of date on election day. People who were not previously registered can show up and vote.
Yes. In my WI township, I remain “registered” if I vote within 4 years. If I ever missed and was removed, I could register at the time of voting. That being said, I don’t think registration lists are exactly meticulously maintained.