psql connection via localhost or 127.0.0.1

I am using postgres 9.4beta3 (Debian jessie)

this is my pg_hba.conf
----------------------
local all postgres peer
local all all md5
host all all 127.0.0.1/32 md5
host all all ::1/128 md5
----------------------

but if I use psql to connect via tcp to the server (on the same machine)
I can see two differente behaviours if I use "localhost" or "127.0.0.1"
If I understand correctly the first two lines are useful only for Unix
sockets and not for tcp-ip connection.

------ with localhost------------
edoardo(at)host:~$ psql -h localhost -U postgres
psql (9.4beta3)
Connessione SSL (protocollo: TLSv1.2, cifrario:
ECDHE-RSA-AES256-GCM-SHA384, bits: 256, compressione: disattivato)
Digita "help" per avere un aiuto.

postgres=#
------------------------------
no password request

------ with localhost------------
edoardo(at)happy:~$ psql -h 127.0.0.1 -U postgres
Inserisci la password per l'utente postgres:
------------------------------
with password request.

Why localhost is not an alias for 127.0.0.1 ?

I did some search on Google but with no useful results

thank you
Edoardo