The first time I saw a premium card I thought that it was like that because it was a Legendary card. Like this:
But I discovered that Legendary cards have a "normal" version:
I thought that maybe it could be explained cause the game is in beta. Like, the normal one was the first version of a Legendary Card, than they became a little more detailed. But searching a little more I found this:
How can I get this kind of card?