Legitimo but then Natural?

I was wondering if any of you have experience on what is going on when certain children of the same parents are labelled legitimate while others are labeled naturales.

For example, the children of Jose Ramos and Anastasia Jimenes, who were living in Sayula, Jalisco in the 1860s.

The two following children are listed as legitimate:

Barvara: https://www.familysearch.org/ark:/61903/3:1:9392-GL94-TX?i=220&cc=1874591&cat=291444
Ma. Genobia: https://www.familysearch.org/ark:/61903/3:1:9392-GP9C-6B?i=465&cc=1874591&cat=291444

But this son is listed as natural:

Petronilo: https://www.familysearch.org/ark:/61903/3:1:9392-GP9X-GY?i=171&cc=1874591&cat=291444

Perhaps I need to look further into Jose and Anastasia to see if they are related, but I was just looking for what immediately comes to the mind of the more experienced researchers here.

Thanks!