Duration in result page

The duration in the result page for my project does not reflect the actual time spent on each activity.
My last activity is just before the result page. If I click back page (means from result page back to my last activity), then go back to the result page, the time difference in duration could be ranging from tens of seconds to hundreds of seconds; but in actual that was only a few seconds I backed to the last activity page.
Is there something I missed out?