Looking at BOD, the cheaper fare (IKV2K) includes a rule regarding length of stay, as below. The single night, more expensive, fare (DKV2K) does not. The more expensive fare does book into D rather than I, but looking at it I don't think this is caused by a lack of availability of I, rather the IKV2K rules.
Category 6: Minimum stay requirements TRAVEL FROM LAST STOPOVER MUST COMMENCE NO EARLIER THAN 2
DAYS AFTER DEPARTURE OF THE FIRST INTERNATIONAL SECTOR.
edit : too slow, beaten by Ldnn1