CONFUSION OVER CASTA: Francisco Becerra & Margarita de Lira, in Nochistlan, Pinos & Cienega de Mata, Ojuelos.

I am confused.

I'm researching my g5's Francisco Becerra and Margarita de Lira, who lived on las Letras in Pinos, and worked as servants at Encinillas. According to their 28 Jun 1716 marriage record, he was Mulato Libre and she India:
https://familysearch.org/pal:/MM9.3.1/TH-1-18622-35857-28?cc=1874591&wc=M99L-ZTJ:234281493

On their 27 May 1716 IM, Francisco says he was 25 and born in Nochistlan, and his parents are Juan Becerra and Teresa de Islas, Mulatos Libres; Margarita was born in Chinanpas, and her parents are Juan de Lira and Maria de la Cruz, casta is not given:
https://familysearch.org/pal:/MM9.3.1/TH-1-18564-63796-52?cc=1874591&wc=M99L-Z5C:1553416362

I found the marriage record for Juan Becerra and Teresa de Islas, married 26 Jan 1682, in Nochistlan; his parents are Francisco Becerra and Josepha Flores, and hers are Cristobal Mexia and Ines de Islas, who were all ESPAÑOLES:
https://familysearch.org/pal:/MM9.3.1/TH-1-15188-1589-11?cc=1804458&wc=M99V-VQY:n651356006

I found the baptism record for Francisco Becerra, born on 20 Aug 1691, in Nochistlan, to Juan Becerra and Theresa Mexia, again, all Españoles:
https://familysearch.org/pal:/MM9.3.1/TH-1-15280-6249-6?cc=1804458&wc=M99V-NCQ:n832064077

Where is the "Mulato Libre"? Nowhere in Francisco Becerra's IM does it say he was adopted or expuesto, and we have his baptism certificate, which clearly says he and his parents were spanish!

On the other hand, I found the baptism record for Margarita Lira, 07 Jun 1688, in Cienega de Mata, Ojuelos, on which her casta is listed as MULATA LIBRE and not INDIA or MESTIZA, as her marriage document and her childrens' baptismal records suggest:
https://familysearch.org/pal:/MM9.3.1/TH-1-18576-47228-53?cc=1874591&wc=M99L-ZPX:n895374894

We all know notaries made errors, especially if individuals weren't present, but how can Francisco Becerra and his parents, Juan Becerra and Theresa de Isla, go from being "Españoles" to "Mulatos Libres," especially on his IM, for which he was present? Perhaps he was dark-complected, and the notary thought he was mulato, and since Francisco couldn't read, he didn't know what the notary had written? That's the only explanation I can think of.

Any thoughts?