play.google.com and Google Play app out of sync
It may be the link to the Play store was lost. I think if you install an app from, say, the amazon app store, Google Play will not be able to update or uninstall it, as the link is gone. Similarly, restoring an app from a previous rom backup may not create that link.

Titanium Backup can recreate the links (the free version can can't do this). Or you could uninstall the app and try installing it fresh from Google Play.