How does Mrs. Jewls end up contradicting herself?

In Chapter 3 Mrs. Jewls tries her level best to teach Joe to count. But no matter how hard she tries or whatever methods she uses she just can't seem to get through. Eventually, she gives up, telling Joe that one day he'll wake up in the morning and suddenly find that he'll be able to count. Sure enough, that's exactly what happens.
But before then, poor Joe has to go through the long, torturous process of learning to count potatoes and pencils. Each time he's asked to count the items in front of him, he gets the right answer. But when he does, he's immediately contradicted by Mrs. Jewls, who in the process also contradicts herself. So for instance, when Joe says that there are eight potatoes, Mrs. Jewls says "No, there are eight," which really confuses Joe, who just wants to go to recess. What she means by this is that, although Joe's given the correct answer, he's counted the potatoes in the wrong way.
One could also say that Mrs. Jewls contradicts herself by teaching Joe to count when all the while she knows that one day he'll wake up and know how to do it without her assistance.