The Convention badge states that it is awarded for
10 posts with score of 2 on meta.
Meaning a user must post at least 10 posts (questions or answers) with a score of 2 or more.
If I understand that correctly, then the most recent user who got the badge has only 8 answers with a score of 2 or up and 1 such question.
That makes a total of 9 qualifying posts. So, where did that 1 other phantom post come from?