Placencia or Robles - or both?

Hello all.

I have found a family in my line that sometimes goes by PLACENCIA, and at other time they are registered as ROBLES.

My 4 gg-mother is MONICA PLACENCIA, married in 1815 to JULIAN ALVARADO in Moyahua, Zacatecas.

I found that on her children's records, she is MONICA PLACENCIA, and her father is JOSE ONOFRE PLACENCIA (her mother is JUANA MARIA DE TORRES).

However--in her marriage record and her siblings' marriage records, they are all DE ROBLES.

Can anyone shed light on this?

Thank you!
-Claudia