I think cavalry should count as having an extra rank per model. Think about it: they have a 25x50 base, instead of a 25x25, on that base there are two separate entities, so in effect it are two models on the same space as two models. So why not?
Further, the RB says that static CR by ranks is meant to represent the pressing of the men behind the first row. A horse (esp. an armored horse) with a man on top weighs much more than 2 men on foot. So in that respect, I think a guy on a horse deserve as counting as an extra rank!