Multiple categories, same topic

Question

While stumbling around in categories, I discovered the existence of several (multiple) could-be duplicate categories:

• United States Navy, sub-category of United States Armed Forces
• United States Army, sub-category of United States Armed Forces
• United States Air Force, sub-category of United States Armed Forces
• United States Army Air Corps, sub-category of United States Armed Forces
• United States Army Air Forces, sub-category of United States Armed Forces
• US Secretaries of the Navy, sub-category of US Department of the Navy, sub-category of US Department of Defense, sub-category of Executive Branch
• US Under Secretaries of the Navy, same hierarchy as above
• US Secretaries of the Army, sub-category of US Department of Defense, sub-category of Executive Branch
• US Secretaries of the Air Force, sub-category of US Department of Defense, sub-category of Executive Branch

This multiple categorization is confusing. Shouldn't these military departments be under the United States Armed Forces category? Could/should the Armed Forces and Department of Defense be merged? Same for US Department of the ___ and the United States ___. 

While I'm on a roll, Executive Branch is very non-descript - which country? I know it is the United States, but that's not the point.

Answer 1

No, Gary, the distinction is between members of actual military forces and politically appointed civilians to oversee military forces.  "Secretaries" are not in the military command structure and the "Department of Defense" is not a military unit.