This is related to a setting on the 'Tarification' tile. 

In the billing period, you can set whether ProPlanner should take into account the start and end time of the reservation, or the actual check-out or check-in.

If '1' is chosen for contract, the reserved check-out will be taken into account. On the other hand, if 'Effectief' is chosen, the actual check-out moment will be taken into account. 

For '2', the same applies as above, but for the check-in.