This is not entirely true: you may have different objects in memory in such a way that both fires rule. In this case, if you had two (or more!) AccountHolders for the same Employment, each of those having different BusinessName's associated, both rules (with and without the 'not') would fire.
The following rule fires both when the “not” is there, and also if the “not” is commented out. Clearly, both cannot be true, so there is something wrong somewhere.
-- "To understand what is recursion you must first understand recursion"