Usually, when I go to
http://gmail.com, I get a login page. But sometimes, I get a page that tells me about Gmail’s features and invites me to sign up (with a smaller link to a login page). I get both pages served to me in a "new" browser (viz, using Firefox's private-browsing mode without having logged in to Google in that session), in the same browser on the same machine as one another. What determines which page I get?
 I can't reproduce the latter at the moment. Therefore, I can't provide interestingly contrastive URLs/screenshots of both the pages — so I'm providing a URL/screenshot of neither.