Books have a long lifespan, so it's useful for the content of a bookseller's website to last a long time also. Since the primary content of this site is created and published as "newsletters", the archives are arranged that way too.
Due to some implementation constraints, the URL at which newsletter sections are displayed when they're the current edition changes when they go into the archive (well, the archive URL is available from the beginning, but the obvious site navigation takes you to a special URL for the current edition). This is not ideal. It causes problems around the time each newsletter comes out, with people following old links getting the new content. It would have been better to do the work to make sure the URL stays the same from the beginning. This also conflicts with there being a constant URL for the latest upcoming SF books section, for example; the solution to that is probably redirection, so that both are constant.