User authentication problem

I have a database in a PC with linux and I try to connect from other PC with
linux too, when I try to do a query from command line with psql and the -u
option :

    psql -d obrascop -c "select * from obras" -hpedidos -u

I get the following error if i suply an user that exists in /etc/passwd

    Connection to database 'obrascop' failed.
    Password authentication failed for user 'postgres'

But if i connect with an user that only exists in PostgreSQL it works
Can someone tell me why

I have the following line in pg_hba.conf
    host         all         192.168.0.15  255.255.255.255   password
where 192.168.0.15  is the host from which I try to connect.


Thanks
Guillermo