Hi Wilfred,
The Visited Web Page filter filters off of the lead's activity. All leads that are in that Smart List should have a "Visited Web Page" activity that coressponds to the specific web page you selected.
It is possible that the Original Source Info is the web page that you are filtering on for many of your leads.
If you find that the Smart List is populating leads incorrectly, please open up a Support Case and we will be able to taken a closer look at your instance.
Thanks!
Will