The posts are listed chronologically from top (newest) to bottom (older) by the time stamp of the last post in the thread. The exception to this are the "Sticky Threads" that are stuck at the top by moderators for informational purposes. By doing this, it keeps "live threads" at the top and lets "dead threads" fall back to successive pages. It seems to be the most logical way to do it, if not, active threads would go further back page-wise as new threads are created, and it would be harder to find active topics. Not to mention that some folks are still on dial-up and it would be a great pain for them to load successive pages, one after the other, in search of active posts.