Assignment data and sort key aren't the same

Hi All Experts,

In the master data of vendor/customer, we entered to the field Sort Key - Posing Date key (the determination - BUDAT).

Reconciliation Accounts also have Posting Key in Sort Key.

But when we post, vendor or customer parked documents from FBV0,

the assignment field receive last numbers from document number.

Also we don't have any deffinitions in transactions OBIA & OBIB.

Why the system receive the last document number's numbers, instead of Posting Date?