Vulnerable item was removed from the Vulnerability group
Filter conditions defined in the VG changed or the VI changed so it no longer matches the conditions from the VG.
If vulnerability group X groups vulnerable items using a filter. All VIs matching the conditions defined in the filter would be associated with the vulnerability group. If the vulnerable item was matching the criteria defined in the filter in the past and was added to the VG but currently either the filter was modified or the VI so it no longer matches the conditions it will be removed from the Vulnerability group.
The 'Refresh associated vulnerable items for non-VGR based VG' scheduled job is responsible for the removal of the VIs from the VG if they do not match the conditions. (