I really don't know - but JK seems to like to emphasise that its not what a person's heritage is that determines what they are going to be (eg a Gryffindor/Slytherin, bad/good wizard etc) but their choices. Harry was going to be put in Slytherin as well although he is a half blood, but he chose not to be.
Also - remember that Riddlle was a descendant of Slyherin. Guess he did not have control over who his descendants married
Its curious - if Salazar thought wizards of Muggle heritage were 'lesser' than purebloods, why would he chose some to be in his house via the Sorting hat?
Really interesting question
