If memory serves, Next Payment Due date is 2 days before actual plan rollover. The intent was that 2 days before actual plan rollover, they would 'sweep' your account to see if there were rewards (now account balance) to cover the payment, and to charge your payment-method-on-file if there was no balance. If your credit card failed to charge, this would give a short window for you to receive notification and correct the issue without a loss of service.
I thought I recalled that early on, the rollover date was indicated instead of the Payment Due date, but not positive of that. In any case, it's been this way for quite a while, and it's better than only listing the rollover date.