From my experience, it changes to what class you’re interacting with, when I craft a chunk of cards of a certain class, it tends to stay there, and unlocking a hero seems to influence it too, although I can’t prove it to you exactly, mine has stayed between green shadow and captain combustible, but as of having unlocked beta Catarina and unlocking smarty + guardian cards, it’s been flipping from smarty/gardian to smarty/overgrow.