Why do some Parmigiano Reggiano contain bacterial cultures?
The bacterial cultures are what the document you quoted from refers to as "fermented whey".  The previous paragraph in the Specification of the Parmigiano Reggiano Cheese from the Parmigiano-Reggiano Cheese Consortium 
 web site reads as follows:


  Fermented whey is added to the milk, which is a natural culture of lactic ferments obtained from
  spontaneous acidification of the whey remaining from the cheese-making process of the previous
  day.


All Paramgiano Reggiano cheeses would have these bacterial cultures in them, not just the cheap ones.