With the password protection feature, a visitor comes to your site and enters the username and password. This will put a cookie on their browser that will store the username and password so they don't have to enter it again the next time.
Anytime someone accesses the password-protected page from a computer that has never accessed the page before, they will be prompted for the username and password.
What you can do if you want to have the password and username reset each time someone leaves the site is have them clear the cache. Here is a link with more information on how to do this: Clear Browser Cache.