The issue first appeared when the Smart App Banner feature was instituted in iOS 6. Smart App Banners allowed developers an easy way to promote their iOS app within Safari by automatically scanning and detecting whether a specific app is on a user's device. If present, the banner invites the user to exit Safari and open the standalone app. If the system does not detect the app, the smart banner will offer a link to download the software from the App Store.
As seen in the example above, Pinterest's iOS app is not installed, thus a banner directing the user to install the app is displayed at the top of the service's web portal.
From the release notes:
Apple released the latest version of iOS 6 earlier on Monday, bringing enhancements to iTunes Match, the ability to purchase movie tickets with Siri, support for more LTE carriers and a host of minor bug fixes and backend improvements.