postgresql unix socket connections

Hi,
I'm trying to understand some issues that I'm having with the unix_socket
settings and pgsql.
I have 2 machines with pg v9.2.5 with the same next settings :
#listen_addresses = 'localhost'
#unix_socket_directory = ''

in both of the machines I run netstat to check on what socket the postgres
listens for connections and I got the same output :
machine 1
netstat -nlp | grep postgres
tcp 0 0 127.0.0.1:5432 0.0.0.0:*
LISTEN 2049/postgres
unix 2 [ ACC ] STREAM LISTENING 12086 2049/postgres
/tmp/.s.PGSQL.5432

machine 2
tcp 0 0 127.0.0.1:5432 0.0.0.0:*
LISTEN 3729/postgres
unix 2 [ ACC ] STREAM LISTENING 51587140 3729/postgres
/tmp/.s.PGSQL.5432

In both of the machines I tried to check if there are some PG environment
variables but nothing was set :
env | grep PG

The pg_hba in both cases is the default pg_hba.

Now, In machine 1 when I run psql I get the prompt password but in machine
2 I keep getting the next error :

psql: could not connect to server: No such file or directory
Is the server running locally and accepting
connections on Unix domain socket
"/var/run/postgresql/.s.PGSQL.5432"?

One important thing that I didnt mention, is that I installed in machine 2
package postgresql-libs.x86_64 0:8.4.20-8.el6_9 from the postgres
repository (in order to upgrade it to 9.6).

I solved it in machine 2 by setting the unix_socket_directory to
/var/run/postgresql/.s.PGSQL.5432 and restarting the database.

My questions are :
1)Why in machine 1, where I dont have a soft link
/var/run/postgresql/.s.PGSQL.5432 that directs to the temp dir I can
connect succesfully ? (env|grep PG didnt show anything).
2)What might explain the issue on machine 2? Or maybe machine2 works
normally but machine1 has an issue ?