There is a fixed date for getting your bill. Then you will pay the normal amount, The second bill is for the days you can travel prior to this date, this bill is lower. So you pay for the remaining days until you're at your fixed date and from then on for a whole month. When you already had a paid subscription, it is partly reimbursed, as you do not use it anymore.

Actually you do not pay more.

Also have a closer look. In a lot of cases it is a bill with a minus sign and that is the amount that is reimbursed.