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(at)marathon bugzilla-2.20]# psql bugs bugs
psql: FATAL: Ident authentication failed for user "bugs"
...
but if I
[root(at)marathon bugzilla-2.20]# su bugs
[bugs(at)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?