Oh.
Well, maybe she did it on purpose... I think Harry HAD to be a halfblood, because his mother needed to be Muggle-born, and his dad needed to be a wizard, too...
And one of the other two had to be Muggle-born, and I'm not surprised it was Hermione because she probably wanted the Muggle-born to be the smartest of the three.
And as for Ron... well, I can't imagine him being anything but a pureblood blood-traitor...