Because our membrane is not only permeable to potassium, as we assumed here, but it's also permeable to sodium. So what will happen? Well, basically, as Our potassium is being pumped to the outside, the sodium will also begin to slowly pump to the inside. Now, the rate at which our sodium is brought to the inside is much smaller, and so that means much less of those sodiums will be brought to the inside. Then the potassiums will be brought to the outside. So the sodium will flow naturally from the high concentration, the outside to the low concentration to the inside.