No good reason.
The correct nomenclature is:
Enhanced Quiet Touch or Quiet Touch = rubber dome
Soft Touch = greased buckling spring
But, that part number is an old-logo Dell regular buckling spring board. My guess as to why it says "Quiet Key" is because of Dell cranking out 11ty billion QuietKeys, and people thinking every keyboard Dell makes is called a QuietKey.
It's essentially a 1391401 with a modified top case with a Dell logo slapped on, and an AT cable (TECHNICALLY making it a 1390131 with the Dell top case, but...)