Surprising (?) Sequence Behavior

Hallo all

During a performance tuning session I had a complex
query that gives some form of ranking. The "correct" way to
solve this, is the use of a scalar subquery that provides
the rank (or use "dense_rank over" in oracle).

But in my case the query is much too slow in this special case.
Even with small number of records that fit into memory (no IO).

So I'am searching for a faster solution and tried
also to use temporary sequences to achieve the same effect.

Example 1:
DROP SEQUENCE IF EXISTS s;
CREATE TEMPORARY SEQUENCE s;

SELECT
    nextval('s'), t.name
FROM
(
    SELECT
       tablename AS name
    FROM
       pg_tables
    ORDER BY
       tablename
) AS t;

gives:
    1 pg_aggregate
    2 pg_am
    3 pg_amop
    4 pg_amproc
    5 pg_attrdef
    6 pg_attribute
    7 pg_auth_members


But if this query is combined with a simple extension it does
not work as expected.

DROP SEQUENCE IF EXISTS s;
CREATE TEMPORARY SEQUENCE s;

SELECT
    nextval('s'), t.name
FROM
(
    SELECT
       tablename AS name
    FROM
       pg_tables
    ORDER BY
       tablename
) AS t
WHERE
    t.name = 'pg_am'
;

The result is:
    1 pg_am

instead of:
    2 pg_am


At least for me this is surprising!
Any hints? Or do I miss something obvious?

thanks a lot, richard