Gaps in AP Invoice Number Created by AR Customer Refund Process

In oracle fusion, Gaps in AP Invoice Numbers (AR Application Reference number while issuing refund in AR) are Created by AR Customer Refund Process.

How are these numbers generated in fusion? Sometimes they are sequential and sometimes they are not. Tried various levels of testing but not getting a clear logic for these numbers generation. We are using the attached setup for document sequencing for our ledger.