Solution : https://service.sap.com/sap/support/notes/585513 (SAP Service marketplace login required)
Key words : 
symptom document number gaps occur, document number gap & 1 occurred due, terms document number gaps, document number gap, internal number assignment, parameter transactions based, batch input runs, implemented sap notes 365096, call transaction se91, message class f5a
Related Notes : 
       
| 607477 | |
| 598940 | |
| 594785 | |
| 428391 | Inexplicable document number gaps in FI (II) | 
| 365093 | Inexplicable document number gaps in FI | 
| 175047 | Causes for FI document number gaps (RF_BELEG) |