Ninjan's right that it was the 2,500'th thread, the fact that it has an id of 22,722 is curious though. How is the id generated I wonder.
nand
A lot of content management system's treat all posts the same in storage. If the forums work the same, then threads, comments, PMs, news, etc, would all be stored the same way (and, thus, be part of the same ID scheme) and separated out when called for.