If you book workouts and make reservations for activities which will take place in the next booking period, they will be charged in the period they are actually taking place in. You will then get the amount of open credits at the beginning of the new booking period. This is the reason why you will get fewer credits at the beginning of the new booking period.
Please note: Unused sports credits from your FLEX subscription cannot be carried over to the following booking period. Any remaining credits will expire at the end of your membership month.