Being short on wristbands isn’t too much of a downer - people can write their names on a list provided by the tour people and they can receive their band in the mail later.
Tours rarely work out as planned - better they are “short” than nobody wanting them.
The wristband is what gets you in to get an autograph. i.e. They promised that 1000 people would get autographed books and only 500 were let in.
Apparently, they are letting people sign up to get an autographed sheet to insert into your book if you weren’t able to get a personally signed copy.