Google Play Store showing Open button instead of Update button

I had a similar problem and it turned out that there is a "glitch" with the update button in that it desappears in landscape mode, and is only visible in portrait mode. When I searched to find out what this was about, I came across this page (we are on), so I thought I'd post my answer here for anyone who might also be stumped and might stumble here.

In order to show an update in Google Play, your app either should have been installed through Google Play or it should be a system application.

If you have installed your application through adb or installed from other sources, Google play will show "Open" instead of "Update".