job run logs for jobs of type stored_procedure are purged immediately

 DB is 11.1.0.7
The scheduler job purge_lob purges too much:
Although global scheduler attribute log_history as well as class atrribute log_history of relevant job classes are =30, the purge job does the following:
- job run logs for jobs of type stored_procedure are purged immediately
- job run logs for jobs of type chain are purged after 30 days as required.

Thanks
Sara