I have used  "No Gap" before itself.  Again it happened, 6 serials missed. No idea where it has gone or why missing.

This usually happens when any invoice is deleted from the system. 

If you are using any automation script to create and validate the invoices, at the time of failures the sequence gets updated but the invoice will not be validated.

To avoid such issues, you can update Sequence Number --> Implementation as  "No Gap" instead of standard.

i think you have posted and canceled invoices.