need clarification on CTE/join

if I do:

postgres=# with numb as(select ceiling(2582*random()) rand, generate_series(1,5) as monnum) select rand, monnum from numb;
 rand | monnum
------+--------
 1543 |      1
 2299 |      2
  205 |      3
  523 |      4
  677 |      5
(5 lines)

ok, fine. The random numbers are at random...and the generate_series are ordered...

I have a table firstnames(id serial, firstname text) with 2582 lines containing firstnames sorted in alphabetical order.

--when I do, with the same CTE:

postgres=# with numb as(select ceiling(2582*random()) rand, generate_series(1,5) as monnum) select monnum, firstname from numb, firstnames where numb.rand= firstnames.id ;
 monnum |  firstname
--------+-----------
      2 | Christine
      1 | Firas
      4 | Firmin
      3 | Rawane
      5 | Titania
(5 lignes)

which mean that what I get is a set of firstnames ordered according to the firstnames table, and NOT to the result of the CTE.

--Now if I cast the result of the ceiling function to int:

postgres=# with numb as(select ceiling(2582*random())::int rand, generate_series(1,5) as monnum) select monnum, firstname from numb,firstnames where numb.rand=firstnames.id;
 monnum |  prenom
--------+----------
      1 | Dexter
      2 | Harrison
      3 | Angilbe
      4 | Narcisse
      5 | Marcel
(5 lignes)

Now its ordered according to the CTE. (and the firstname list is at random)

I did test the same thing after putting the result of the CTE in a table, with the very same behaviour.

So.. I would like to understand the "why" of this behaviour, ie. the change of order when I do the cast.

(The original reason was that I was creating a test env with millions of rows with this kind of CTE and was quite surprised to discover that the result table was ordered...which was not at all my goal)

thanks,