The developer has made the AI decks himself based on synergies and/or themes.
The Witch plays only big cards with 13 power and tries to shuffle them into your deck, which not only puts you at a disadvantage but also fits the theme of a witch (13 is an evil number). Alucard uses a hearts deck with a Blood Donor Card that heals him when he plays hearts cards, which also fits because he's gaining health by draining power from his cards like a vampire.