This probably goes back to the original meaning of "secretary". The word is of French origin, and was applied to a person who was privy to the "secrets" of a noble or king. The Secretary was the person with whom one "secreted" private information and communications. The term has been carried into modern usage as an official in charge of a particular branch of government...and the one who knew all the "secrets" of that office or position. The word is also used in modern times to describe an elaborate desk with many compartments...which contain all of the owner's personal information and "secrets".