9 isnt a prime number
^Right! -don't really know what I was thinking there
Here's the thing: Seven is an ancient lucky number. Nine is this extra magic you get when you have three threes and three is the most magic number of all (the Christian god is comprised of three deities - three brothers given each three tasks and the winner takes the princess and half the kingdom -that
magic). Thirteen is also an ancient lucky-number.
The expression "7-9-13" apparently is rather new: it was first found in print in 1913.