This is because you are using FrontPage 2003, Expression Web, or Dreamweaver, and the page you are editing is attached to a DWT (Dynamic Web Template).
The use of DWT technology allows you to have a common template or layout which a number of pages use. Update that one layout, and all pages attached to that DWT will change accordingly.
Look within the “Templates” folder inside your website template and identify the .dwt file that is attached to your page.
If you do not know which .dwt is attached to your page, open up the page that you are trying to edit, switch to CODE/HTML view and scroll to the top of the code.. look for a line that looks like:
This will tell you which DWT (master layout) you will need to edit.