Gaps in PSTGNMBR in GL20000

Understood that there's a sequential PSTGNMBR (Posting Number) assigned to each posted JRNENTRY, under what circumstance that there's a gap in the PSTGNMBR?

i'm observing 2 Gaps at least in the entire GL20000 table. Though running the Reconcile Posting Numbers can help close the gap, I'd need to know what could have caused the Gap in the first place, to answer an audit finding.

Thanks n cheers.