Check out this blog post for a lot of great info on what rematching is and what rematching is not.
In a small percentage of cases, people do not receive their initial gift. It is horrible and we hate it but it sometimes happens. People may have had problems with mail or theft issues, their Santa may have had unforeseen circumstances, or sometimes the Santa is just being a Grinch. Reddit Gifts does not like Grinches. Grinches are banned. However, if your initial gift does not show up, you may be rematched.
When a person does not receive a gift from their original Santa, we try to rematch them with another person who generously volunteers to be a Rematch Santa. That match, like the original matching, is arranged randomly.