Window function order changing order of whole query

I tried posting this on the pgsql-sql but they don't seem to be posting
successfully there, so I'm re-posting it here.

I've had a look at examples of lag and lead window functions with order by
inside the OVER clause, and I'm confused as to why it influences the overall
order in the output.

For example (for a table called category with incrementing id numbers up to
26):

SELECT id, lag(id) OVER (ORDER BY id DESC) FROM category;

This would yield:

id lag
26 NULL
25 26
24 25

This is the equivalent of what we would get with:
SELECT id, lead(id) OVER (ORDER BY id ASC) FROM category ORDER BY id DESC;

I would expect the row order in the results not to be influenced by the OVER
clause's ORDER BY as I thought that was just to determine how the aggregate
window function's values would be output. It's almost as if the main part
of the query has inherited it's order from the OVER clause.

Am I missing something here?

Having a look around, it looks as if Postgres might be misbehaving.
According to this page,
http://my.safaribooksonline.com/0596004818/sqlnut2-CHP-4-SECT-3, the ORDER
BY in the window function's clause shouldn't be having this ordering effect:

"Furthermore, the order within these groups is defined by an ordering
clause, but that order only affects function evaluation, and has no effect
on the order in which rows are returned by the query."

The behaviour is unexpected from my perspective, but obviously there are
workarounds. Is anyone able to confirm any of this?

Thanks

Thom