Keep activity for each user doesn't work

Hi, I have test the 6.x-4.x-dev module and make sure that I setted Keep 10 activity for each user when Delete messages older than one months. But when the activities older than one months, cron will delete all user's activity. The Keep activity for each user setting doesn't work.
I read the mysql code about:

db_query("SELECT        t1.uid,        t1.uaid as 'uaid',        COUNT(*) as 'rows_per_user',
        t1.timestamp as 'real_date',        MIN(t2.timestamp) as 'oldest_date',        count(t2.uid) AS 'count'      FROM {heartbeat_activity} AS t1
      INNER JOIN {heartbeat_activity} AS t2 ON t1.uid = t2.uid AND t2.timestamp >= t1.timestamp
      WHERE (t1.timestamp, t1.uaid) < (t2.timestamp, t2.uaid)
      AND t1.message_id NOT IN (" . db_placeholders($unlimited_templates) . ")
      GROUP BY t1.uid, t1.uaid HAVING COUNT(t2.uid) <= %d
      ORDER BY t1.uid, t1.uaid, t1.timestamp DESC", $arguments);

But I have no idea where is wrong, Is it a bug? Or anyone can help?