Cancelling a join your Mac turned down frees the seat
When the team let you in but your Mac turned the answer down, cancelling now tells the team, so you are not left listed as a member who never arrived.
Your Mac checks the answer a team sends when you join. If something did not match, it stopped, but the team had already added you. Cancelling then told nobody, so the team kept a member who never arrived.
Now cancelling tells the team in that case too, and the seat is freed. When the team itself said no, nothing changes.