Consistency is not the same thing as simplicity. Simplicity refers to how many rules a game has, and how easy it is to understand each individual rule. Consistency refers to how well those rules mesh together.
It follows that even a very complex game can be perfectly consistent. However, in practice, very consistent games tend to be simple, because the more rules a game has, the more difficult it becomes to get them all to follow a deep pattern.
It's also possible for a game to be so simple that the concept of consistency hardly applies. If a game has only one rule, there's nothing with which that rule can be consistent! Gomoku comes to mind in this regard.