couple possibilities I can see here:
1. There have been a number of cases of someone hacking arena and doing things like changing people's decks around, so could be something to do with that although usually when this had happened, they left a completely different deck with something like 1337x mark etc.
2. You were left with your oracle card from last week...were you still using this same card in the deck at the time? If so, exactly how many days old was the deck? Could be the deck gets removed or reset in some way after a certain amount of time or that this is what happens if you're playing against it at the time it drops out of the top 500 and is no longer active?
