The presence of wild cards brings a new element into the game, allowing a player to use such a card to stand for any card of the player's choosing. It increases the chances of drawing more valuable hands. It also potentially alters the ranking of different hands. One can even draw a five of a kind, which typically goes to the top of the rankings. That's a striking result.
Wild cards increase the number of ways in which each type of hand can occur.
The amount of that increase depends on which cards are designated as wild. For example, with deuces wild, four of a kind occurs more than twice as often as a full house. So, modifying the rules to rank a full house higher than four of a kind might produce a more consistent result. A player, however, often has a choice in how to declare a hand, and that choice will invariably produce the strongest possible combination according to the declared rules. The authors examined several wild-card options and found that the standard ranking proves to have fewer inconsistencies than other possible ranking schemes.