In other words, when you think you are going straight and pointing your skis downhill and parallel, you are actually turning them both towards each other thanks to the edgepressure of your natural stance.
If I roll them evenly, this disproportionate edgepressure remains, and at a high speed the skis bend into two differently shaped arcs, rather than ideally parallel twins, meaning they are making two different turns of different radii and different speeds, and occasionally this results in a seemingly unexplainable fall.
But since I pronate, if I was not in neutralizing boots, I would already have artificially more pressure on the left edge of the outside (right) ski and not enough on the left edge of the inside ski.