Set returning functions in select column list

* Apologies if anyone receives this twice. I previously sent it from
another address and it did not appear to go through.

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