When configuring archive rules you can create additional rules to handle related records.
For example, you can set up a rule to delete related records of a record that is being archived.
Once a record is deleted via one of these rules it will be logged in 'sys_audit_delete' unless 'no_audit_delete=true' is defined for its parent table.
This is expected behaviour as generally all records are audited for deletes unless the 'no_audit_delete=true' attribute is present on the parent tables collection record in 'sys_dictionary'.
If you do not want deletes to be audited on a table please review 'KB0725007 - Disable auditing of deleted records on a specific table':