Something I've noticed is because of the minimum 20 card deck size.
If you have 20 cards in your deck, and merge two cards in your deck, the game will pull a card from your inventory to fill that empty spot.
That is, after merging you have 19 cards. This is an invalid deck size. So the game pulls a random card from your inventory to bring you back up to 20.
I wonder if Sirop thinks the Cow Curator appearing in their deck is "extra" when they may find that it came from inventory.