Copying data from one table to another

Consider the following two tables:
customer:
id - integer - not null default nextval('customer_id_seq'::regclass)

and

order:
id - integer - not null default nextval('order_id_seq'::regclass)
customer_id - integer - not null
"$1" FOREIGN KEY (customer_id) REFERENCES customer(id) DEFERRABLE
INITIALLY DEFERRED

There are about 30,000 rows in customer table.
Order table is empty.

I am trying to add rows to the order table, one per customer. Here is
the SQL I am using to populate the data:

INSERT INTO "public"."order" (id, customer_id)
SELECT id, id FROM "public"."customer";

The query runs just fine. After execution I found that a minority of
rows in order table have *different* values for id and customer_id
(around 3,000 out of 30,000). Given my SQL I expected the id and
customer_id values to be the *same* for all rows.

Why does this happen? Am I doing something wrong?  Is there a way to
copy data and have the id and customer_id values turn out to be the
same for all rows?

I am using Postgresql 8.3 on Ubuntu Hardy.