Set returning functions in select column list

Can someone explain how set returning functions in a select clause work?

It seems that it is doing some sort of implicit cross join.

deliverance_development=# select id, generate_series(1, 3) from users;
  id | generate_series
----+-----------------
   0 |               1
   0 |               2
   0 |               3
   1 |               1
   1 |               2
   1 |               3
(6 rows)

But if multiple set returning functions that return the same number of
rows are in the same select it doesn't further cross join it.

deliverance_development=# select id, generate_series(1, 3),
generate_series(4, 6) from users;
  id | generate_series | generate_series
----+-----------------+-----------------
   0 |               1 |               4
   0 |               2 |               5
   0 |               3 |               6
   1 |               1 |               4
   1 |               2 |               5
   1 |               3 |               6
(6 rows)


But if the set returning functions return a different number of rows
then it goes back to a cross join.

deliverance_development=# select id, generate_series(1, 3),
generate_series(4, 5) from users;
  id | generate_series | generate_series
----+-----------------+-----------------
   0 |               1 |               4
   0 |               2 |               5
   0 |               3 |               4
   0 |               1 |               5
   0 |               2 |               4
   0 |               3 |               5
   1 |               1 |               4
   1 |               2 |               5
   1 |               3 |               4
   1 |               1 |               5
   1 |               2 |               4
   1 |               3 |               5
(12 rows)


I really don't understand what is going on here. I have checked Google
and the PostgreSQL docs, but it appears either I do not know the key
words to search for or it is sparsely documented.

--
Jack Christensen
jackc@hylesanderson.edu