CAUSE

Oracle decides what rows need to be purged based on the retention policy. There is a special mechanism which is used in the case of the large AWR tables where we store the snapshot data in partitions. One method of purging data from these tables is by removing partitions that only contain rows that have exceeded the retention criteria. During the nightly purge task, we only drop the partition if all the data in the partition has expired. If the partition contains at least one row which, according to the retention policy shouldn't be removed,  then  the partition won't be dropped and as such the table will contain old data.

If partition splits do not occur (for whatever reason), then we can end up with a situation where we have to wait for the latest entries to expire before the partition that they sit in can be removed. This can mean that some of the older entries can be retained significantly past their expiry date. The result of this is that the data is not purged as expected.

SOLUTION

A potential solution to this issue is to manually split the partitions of the partitioned AWR objects such that there is more chance of the split partition being purged.You will still have to wait for all the rows in the new partitions to reach their retention time but with split partitions there is more chance of this happening. you can manually split the partitions using the following undocumented command:

alter session set "_swrf_test_action" = 72;

To perform a single split of all the AWR partitions.

1.  Check the partition details for the offending table before the split:   
    SELECT owner,
     segment_name,
     partition_name,
     segment_type,
     bytes/1024/1024/1024 Size_GB
   FROM dba_segments
   WHERE segment_name='WRH$_ACTIVE_SESSION_HISTORY';
2. Split the partitions so that there is more chance of the smaller partition being purged:
   alter session set "_swrf_test_action" = 72;
   NOTE: This command will split partitions for ALL partitioned AWR objects. It also initiates a single split; it does not need to be disabled and will need to be repeated if multiple splits are required.
     
3.  Check the partition details for the offending table after the split:      
   SELECT owner,
     segment_name,
     partition_name,
     segment_type,
     bytes/1024/1024/1024 Size_GB
   FROM dba_segments
   WHERE segment_name='WRH$_ACTIVE_SESSION_HISTORY';