Hi,

While assigning a sequence number to a record, the 'no gap' sequence implementation ensures that each previous sequence number has been assigned already While this sequence implementation will not skip any sequence number upon assignation, there can still be gaps in the sequence if records are deleted.

Check this link

https://github.com/odoo/odoo/commit/b5b7396b84fedd02535a5162632e749a1a06c0b5