OK ... obviously I should explain how this works.
There are official CentOS public mirrors, there are listed here:
http://www.centos.org/modules/tinycontent/index.php?id=13
(There are 127 mirrors now, 112 of which are up2date and shown in the list)
There are also internal CentOS servers that are not published to general public ... they are used to update the public mirrors.
Mirrors get to be mirrors because they have asked to be listed. These public mirrors (and SOME of our internal centos mirrors) are the universe of mirrors that are available to our mirror system.
Some ISP mirror (who is not official) that one might know about is NOT part of our update system.
When someone asks for the mirrorlist, they are given all the local mirrors (that is, the ones inside and close to their country). As in the example given before ... Australia (au) there are 3 listed mirrors ... 2 of the 3 were passed in the mirrorlist (planetmirror has some content wrapping it that causes issues with our mirrorlist system).
Also included as close is Singapore (2 mirrors listed).
Since you have 4 listed mirrors, we backfill with 6 CentOS mirrors to make the total 10 (in other countries, there will be no back filled centos mirrors). The backfilled CentOS mirrors should not be first on mirrorlist (if fastestmirror is used) as the other mirrors should be closer.
fastestmirror works by making a socket connection to each mirror and timing the connection ... then ordered by time (fastest listed first).
This means that you have 10 mirror failover, and should have the closest mirrors (again, from the above link) to your individual PC.
If a mirror becomes outdated, it is removed from the list.
This allows us to use the public mirrors for updates, which means that there is enough bandwidth for everyone ... and it scales as we add mirrors.
The centosa -> centosc2 are not public mirrors, they are used as filler mirrors only. In the case of au, they would only be used if they were faster to you ... not likely unless the 4 mirrors that are closer are overloaded. In that case they would be used.