Ident authentication failed without su to user

I've created a database "bugs" and a user "bugs" (postgresql user as
well as unix) and even made user "bugs" not require a password but I
still get this:
[root@marathon bugzilla-2.20]# psql bugs bugs
psql: FATAL:  Ident authentication failed for user "bugs"
...
but if I
[root@marathon bugzilla-2.20]# su bugs
[bugs@marathon bugzilla-2.20]$ psql bugs bugs
Welcome to psql 8.0.7, the PostgreSQL interactive terminal.

Type:  \copyright for distribution terms
     \h for help with SQL commands
     \? for help with psql commands
     \g or terminate with semicolon to execute query
     \q to quit

bugs=>

it works.  any ideas why?