You must specify an ORDER BY clause (or OPNQRYF KEYFLD parameter) to guarantee a particular ordering of the results. Before parallel access methods were available, the database manager processed table rows (and keyed sequences) in a sequential manner. This caused the sequencing of the results to be somewhat predictable even though an ordering was not included in the original query request. Because parallel methods cause blocks of table rows and key values to be processed concurrently, the ordering of the retrieved results becomes more random and unpredictable.
An ORDER BY clause is the only way to guarantee the specific sequencing of the results. However, an ordering request should only be specified when absolutely required, because the sorting of the results can increase both CPU utilization and response time.