Recently I applied new layout which was suggested by YouTube. After this my filter option did not work properly on YouTube, i.e. when I filter with LONG (>20 min) category I still found videos having length of 3 min and 4 min also in my search result.
Probably just a temporary issue , if the layout is the cause then you have to remove it.
Solution: Remove New Youtube Layout
http://ml.techyoshi.com/index.php?topic=171.0
How To Switch Back To YouTube’s Old Page Design
http://www.ghacks.net/2012/01/09/how-to-switch-back-to-youtubes-old-page-design/