Strange behavior of "=" as assignment operator

test=# create table foo ( modified timestamp not null default now(), some_value integer );

CREATE TABLE

Time: 22.600 ms

test=# CREATE OR REPLACE FUNCTION public.fn_update_modified_timestamp()

test-#  RETURNS trigger

test-#  LANGUAGE plpgsql

test-# AS $function$

test$# begin

test$#     NEW.modified = now();

test$#     return NEW;

test$# end;

test$#  $function$;

CREATE FUNCTION

Time: 10.429 ms

test=# insert into foo (some_value) values (5);

INSERT 0 1

Time: 1.979 ms

test=# create trigger tr_foo BEFORE UPDATE ON foo FOR EACH ROW EXECUTE PROCEDURE fn_update_modified_timestamp();

CREATE TRIGGER

test=# select modified, some_value from foo;

          modified          | some_value

----------------------------+------------

 2013-05-28 15:41:33.338463 |          5

(1 row)

test=# update foo set some_value = 6 where some_value = 5;

UPDATE 1

test=# select modified, some_value from foo;

          modified          | some_value

----------------------------+------------

 2013-05-28 15:44:51.082989 |          6

(1 row)