I'm not saying I'm wrong or right (and I'm oversimplifying it a lot), but it seems to me as though this can be explained by looking at what's happened with primary, high school, and college curriculum over the years. I'd wager that liberals buy in to what they're taught and so they don't have to come up with a counterpoint, ever. I'm not saying that liberals are smart or dumb because of it, just probably prone to accepting what their schools have taught them.
If someone doesn't accept the curriculum, they know it, and understand it, but they've formed a different viewpoint. Therefore, they can explain both sides pretty well.
Now I will go read the whole article.