Why metric "Redo Generated Per Sec" has different sample interval time

SQL> set linesize 170 pagesize 500

SQL> col metric_name format a40

SQL> col group_name format a40

select s.begin_time,s.end_time,s.metric_name,m.group_name,value,INTSIZE_CSEC from v$metricname m,v$sysmetric_history s where s.metric_name=m.metric_name and s.group_id=m.group_id and s.metric_name='Redo Generated Per Sec' order by begin_time;

SQL>

BEGIN_TIME        END_TIME          METRIC_NAME                              GROUP_NAME                                    VALUE INTSIZE_CSEC

----------------- ----------------- ---------------------------------------- ---------------------------------------- ---------- ------------

20141123 21:23:49 20141123 21:24:50 Redo Generated Per Sec                   System Metrics Long Duration             398.735441         6010

20141123 21:24:50 20141123 21:25:50 Redo Generated Per Sec                   System Metrics Long Duration             417.301614         6011