Actually their frame rates are 1,2 and 3 second. You can check the instance variable.
Because of it at every 2 seconds, upper 2 sprites firing at the same moment (because 2 dived to 2 and 1), at every 3 seconds, top and bottom sprites firing at the same moment(3 divided to 3 and 1), and every 6 second all of them firing at the same moment (because 6 divided to 1,2 and 3)
If you choose different rates you can see it easily. For demonstrating i uploaded a second one