Looking at the configuration management log it appears that since the dawn of time (13th June 2003) that topics and weeks have been separate from a formats point of view. The person to answer this will be the man himself, Martin.
For the most part the code is the same only with subtle differences dotted around the place. A while back I merged the Collapsed Weeks functionality into Collapsed Topics as noticed this very same thing. This saves a shed load of maintenance for the sake of a little switching based on a setting.