Unexpected sort order.

Shouldn't the results of this query shown here been sorted by "b" rather than by "a"?

I would have thought since "order by b" is in the outer sql statement it would have
been the one the final result gets ordered by.

li=# select * from (select (random()*10)::int as a, (random()*10)::int as b from generate_series(1,10) order by a) as x order by b;
a | b
---+----
0 | 8
1 | 10
3 | 4
4 | 8
5 | 1
5 | 9
6 | 4
6 | 5
8 | 4
9 | 0
(10 rows)

Changing the constant from 10 to 11 in either but not both of the
places produces results I would have expected; as do many other ways of
rewriting the query.

Unless I'm missing something, it seems the way I wrote the query creates
some confusion of which of the two similar expressions with random()
it's sorting by.