Claudia,

You have found an example of the beautiful confusion of the Spanish surname system that existed before the standardization of surnames introduced under the Bourbon reforms. Basically, in Spain and Spanish America, it was quite common to give children maternal surnames and sometimes even maternal grandmother's surnames (I have seen many examples of girls being named for their grandmother including her surname and thus having a different surname than all of their siblings).

So in this case, it seems that somewhere down the line, someone could not decide which surname to use and switched back and forth (or perhaps the priest could not choose and switched back and forth). This was quite common and you will run into it more the further back you go. You should stay open to looking for both surnames. It could be that Monica's father used Placencia and Robles and therefore the records for his daughter simply matched whatever name he was using at the time.

Of course, there are also plenty of examples where priests just "got it wrong" and confused one person for another or just put down the wrong name.