It gets the systems of primogeniture mixed up. Equal primogeniture passes to the oldest child, regardless of gender. Cognatic primogeniture passes to the oldest son unless there are no sons, after which it goes to the daughter. Agnatic primogeniture passes to the males only, and if there are no sons it passes to a brother instead of a daughter.
As there are only three female rulers, it seems strange that it would be equal primogeniture, for this would mean that a son was born first 80% of the time. Do any sources specifically state that equal primogeniture is used? If they don't, it makes sense that cognatic primogeniture was used; then, most of the time a son would succeed, unless there were no sons, in which a daughter would succeed.