What do the icons next to defeated opponents mean?

When you defeat an opponent, the game places a weird-looking sigil next to the opponent's portrait:

The colors might be hard to see, but the one to the left is bronze, and the two right ones are silver.

What do they mean? Do the colors represent the opponent's strength? How handily I trounced them? Something else?

Note: this is for Magic: The Gathering: Duels of the Planeswalkers 2012

The icon tells you what difficulty you have beaten that planeswalker on:

• Bronze is Mage
• Silver is Archmage
• Gold is Planeswalker

So, I'm not 100% on this, but my understanding (from the first game) is that the Icons indicate a rating of how well you've defeated that opponent. They go from Bronze to Gold and are a function of remaining life, remaining cards in deck, and number of turns played.

Unfortunately, I haven't played the 2012 yet, so I can't confirm this for the second game