The store product is the package which contains nothing but the game. The latter was for some reason given out with Amazon Prime subscriptions.

It's the same situation with Oblivion. Presumably these packages are remnants from back when buying any Bethesda game would automatically add Arena and Daggerfall to user accounts.

This would be neatly solved if the store would check if every item in a package is owned, then mark the package as owned. This would fix the issue for multiple games.