The 'top level' for the document is a book container and so does not have a topic associated with it. The next level down is the first topic in the document and so acts as a book container and a topic.
Because Microsoft's HTMLHelp is quite complicated to produce volume results quickly we have had to make some basic layout decisions to make it work with the minimum of effort for the user. However once built the .HHC and the .HHK file can be refined with the Help workshop (HHW.EXE)