I have noticed a pattern - there are a lot of surnames that have very similar surnames in the same area. For example, in Polizzi and Petralia, we have:
Cirillo, a noble family Ciraulo, a peasant family
Barranco and Barrancotto
Borgese and Borgesano
Brancato and Brucato
Curatolo and Curto
Ferrante and Errante
Gagliardo and Gagliardotto
Geraci and Giresi
Marzullo, Mazza, Mazzola
Notaro / Norato
Scarpa / Scarpinato
These are definitely not variations of spelling names. In some cases, these are very different families, they marry very different people, and you never see the ancestor of one using the name of the other.
We have surname pairs such as Allegra / Gallegra, Saguto / Santo, Rannazzo / Randazzo etc are completely different. People with these names can be recorded with either surname, sometimes the same person will have one name on their birth and the other on their marriage.
Do these surnames that are not interchangeable have common literary origin and nothing else? For example, Cirillo could be descended from a wealthy lord named Cyril while Ciraulo was named after an unrelated peasant named Cyril who worked on a nearby property.
Could these surnames be branches of the same family? I have noticed that usually the name that seems diminutive has lower social status. For example Gagliardo could have been a noble family and kept the name among children who inherited, while those relatives who inherited little or nothing took 'Gagliardotto'.
The surnames Brucato and Brancato are related in that they are both botanical names derived from forest trees, according to Prof Joseph G Fucilla in his book on Our Italian Surnames (p. 91). Brucato is derived from the tamarisk tree or shrub. Brancato is derived from the willow family (Sicilian from Low Latin blanchia). Brucato, however, can also be derived from an article of clothing, or brocade (p. 187)