The Formative Cycle is the birth month reduced to a digit, and it therefore takes as its raw material one of the more arbitrary units in common use.
The Gregorian months are not a natural partition of the year. Their lengths are irregular for reasons of Roman political history rather than astronomy, and July and August are long because they were named after men who wanted them to be. Nothing in the solar year marks the boundary between one month and the next, and a birth on the last day of April and one on the first of May are a day apart and two cycles apart.
The consequence for this position is a set of hard edges in the wrong places. The Formative Cycle changes at midnight on the first of the month, and the reduced value jumps from nine to one between September and October, so two children born a day apart are given cycles at opposite ends of the range.
An astrological reader will notice that the tropical zodiac faces the same problem and answers it differently, by dividing the ecliptic into equal arcs measured from the equinox, which is an astronomical event rather than an administrative one. The signs therefore cut across the months. This system does not do that; it takes the calendar as given.
Lunisolar calendars make the position harder still, since their months track the moon and slip against the solar year, so a person's birth month depends on which calendar the record was kept in.