As people have posted above; probably for 2 reasons.
[*] "mirc613" is a known trojan virus that was spread to unsuspecting users several months ago.
[*] silly superstition. same reason elevators don't stop at the 13th floor.
You will always find the latest version of mIRC, here
PS. Thank you Khaled. We all love v6.14 and can't wait to see what you have in store for us next!