a "heads-up" on the surnames Herrera and Santiago

In my Jalisco family branch ancestor, the surnames Herrera and Santiago are so intertwining and tricky-confusing.

Most of the earlier ancestors use Herrera de Santiago as a surname.

However, the monkey wrench, many descendants, even in the SAME family, use one or more of these three variations:

1. Herrera de Santiego

2. Herrera

3. Santiago

In addition, many individuals are identified with any or all three variations in different Church records:

i.e. in the Baptism Record that individua's surnamel is HERRERA and/or in the Marriage Record it's SANTIAGO and/or in their children's Records that same individual's surname is HERRERA DE SANTIAGO.

I thought that perhaps the earliest ancestor's surname that used HERRERA DE SANTIAGO also used the mother's surname, BUT, that was NOT the case (so far).

It appears that the surname in the baptism, marriage, or death record on the very same individual of any generation is pure happenstance!