Yeah I get why if you have a Muggleborn parent and a Pureblood parent why you would be pureblood. I hate to say this, but you got to kind of think like what that Nazis did about the Jews. Any inkling of coming from a muggle family then you wouldn't be considered pureblooded. That's why grandparent's blood matter so much as well. During the Holocaust if you had just one grandparent that was Jewish, you were considered Jewish and had "tainted" blood. So, if one grandparent was a muggleborn, you wouldn't be considered Pureblood.

I think that's what J.K. was thinking when she went into such a depth viewing of blood status, especially in Deathly Hallows. You can see so many parallels to Nazi Germany and what took place in that book, such as rounding up the Muggleborns and questioning their blood status. It's disgusting. And as a side note, I found it most appalling when the movie took it a step further when Bellatrix wrote "Mudblood" on Hermione's arm, like the number tattoos the Jews got during the Holocaust.