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
orderby 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.