Unix domain socket inconsistency

Hi all,

I'm a bit confused by my development set up.  I can connect to
PostgreSQL using unix domain sockets by not specifying any host with
psql, and the same applies to vacuumdb, createdb and dropdb.  However,
when I go to use pgbench, it seems to be looking in the wrong place
for the domain socket:

thom@swift ~/Development/postgresql $ psql postgres
psql (9.4devel)
Type "help" for help.

postgres=# \q
thom@swift ~/Development/postgresql $ createdb pgbench
thom@swift ~/Development/postgresql $ pgbench -i -s 20 pgbench
Connection to database "pgbench" failed:
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.5488"?

The actual location is /tmp/.s.PGSQL.5488:

thom@swift ~/Development/postgresql $ pg_ctl stop
waiting for server to shut down....LOG:  received smart shutdown request
LOG:  autovacuum launcher shutting down
LOG:  shutting down
LOG:  database system is shut down
done
server stopped
thom@swift ~/Development/postgresql $ psql postgres
psql: could not connect to server: No such file or directory
Is the server running locally and accepting
connections on Unix domain socket "/tmp/.s.PGSQL.5488"?

What am I missing?

--
Thom