I stumbled upon a post in the Joomla forums about speculation surrounding the mysterious origin of ID 62 for the default Joomla Super Administrator.

Fortunately, and many years ago, I pulled down all the historic downloads for Mambo (from which Joomla spooned, I mean spawned) and decided to look through them. The earliest version of Mambo I had was from 30 April 2001 being what I think was version 3.0.0. The install script for this version has the default user ID set to 1. This is the same for the next five versions but in version 3.0.6, released around 27 July 2001 we see the first introduction of the now famous ID 62 for the Super Admin. Version 3.0.7, released in November 2001, and then the first of the 4.0 series, released a year later in November 2002, carried on the tradition (I joined the project in February of the following year).

So what changed? Well, one hint I found was that there was a significant amount of work done on files that equate to today's user component. Between 3.0.5 and 3.0.6 the developers had decided to encrypt the password and touch up a few things. What I suspect happened was a lot of testing went on between these versions (mind you, over only seven days as 1.0.5 had only just been released on 20 July). The developer cleaned out the test users progressively and the last one left as he/she made the database dump just happened to be ID 62. Another possible is that it was a result of a security exploit that they were fixing (an attacked could easily get into the Administrator). Either way, from there it just stuck.