Is it one particular
Miriam who became Pucci or is it some kind of habit? There's a tendency amongst Italian women (mainly aged and upper class, for what I have observed) to stick all their lives with some childhood nickname , at least in familiar contexts, so it could be that the
Miriam in question was called Pucci by her siblings or parents as a baby, and it just stuck. Examples in my family include a
Maddalena known as Tuti (apparently what she called herself when learning to speak) and a
Francesca known as Nanni (apparently what she was calling her sister
Giovanna, which somehow got back to her).
Francisca as
Paca is a mystery. It works for men as well since
Francisco Franco was nicknamed "
Paquito". In Italy
Paco is used as a nn for
Pasquale.