Solution : https://service.sap.com/sap/support/notes/19050 (SAP Service marketplace login required)
Summary : 
      When creating billing documents via VF04 or during background processing, users may encounter irregular number assignments. The gaps are due to update terminations visible in syslog, buffering of the RV_BELEG number object (checked using Transaction SNUM), and errors that occur between number assignment and the COMMIT WORK command. A common cause for these disruptions are customizing changes made to programs post-number assignment, leading to execution terminations, such as during output determination. To avoid this, review customizations and ensure continuity in sequencing without interruptions.
Key words : 
create billing documents, rv_beleg number object, number assignment, update terminations, transaction snum, error occurs, commit work, customizing activities, occasionally change, termination occurs
Related Notes : 
       
| 175047 | Causes for FI document number gaps (RF_BELEG) | 
| 24279 | Missing billing document numbers in VBRK |