Actually, Apple has always traditionally reserved uids < 500 for its own use.  We simply haven't passed 100 yet. :-)

- Jordan

On Apr 7, 2007, at 11:52 AM, source_changes@macosforge.org wrote:

nextuid and nextgid now return the first uid/gid > 100 rather than the max + 1
This means that new users created with adduser are not visible on the login screen.
Also it was kinda silly to create a new user/group with id 502 when 100-499 were available