Collectible Card Game Headquarters

As I've played, I've noticed random cards that I own (that aren't in my actual deck) are seemingly sometimes in my deck? I cannot find a rhyme or reason for this, my deck is 60 cards. It is quite annoying, as I cannot even play them due to being outside my deck's colors. Is there something that causes this? Thanks.

It's caused by liberating cities taken over by one of the wizards. The ante card you get for winning the duel - which is easy to forget about, since it's only shown before the duel; you get the "[Cityname] is freed! The people rejoice." screen instead of the normal "Won these cards" screen at the end - gets put into your current deck instead of just your collection. IIRC, it gets removed the next time you look at the deckbuilder.

That's interesting. Thank you so much! This had me completely baffled for a long time.