In a small percentage of cases, some people do not receive a gift from their original Santa. If you do not receive a gift from your original Santa, 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 has generously volunteered to be a Rematch Santa. That match, like the original matching, is arranged randomly.
Check out this blog post for a lot of great info on what rematching is and what rematching is not.