When you upgrade an additional student to Premium, you reset the subscription start time on all students. That means you have to pay for the additional time you've added on to the end of the original students who had Premium. This is to keep billing cycles manageable and predictable. Let's say you upgraded Susie in June, then added Billy in September. You now have to pay for Susie from June through September, because you're adding it on to the end of her membership. Thus the additional charge.