Question About Serializable

The documentation states that "concurrent execution of a set of
Serializable transactions is guaranteed to produce the same effect as
running them one at a time in some order."

I'm not sure how the following behavior fits that definition. (Note that
this is just an experiment, not a use case. Purely academic.) I run these
transactions sequentially, and I get a different result than if I run them
concurrently.

This is in 9.3.0

First, I set up a table.

create table x (value int);

Then I run the following transactions. If I run them sequentially, in
either order, I get one row in table x. If I run them concurrently, I get
no rows in x.

It seems like one of these should error out and not commit, so I must be
missing some stipulation.

Feel free to repeat this result.

=========
begin;

set transaction isolation level serializable;

create table z ();

select pg_sleep(5);

insert into x (value)
select 0
where exists (select relname from pg_class
where relname = 'y')
and exists (select relname from pg_class
where relname = 'z');
commit;

=========
begin;

set transaction isolation level serializable;

create table y ();

select pg_sleep(5);

insert into x (value)
select 0
where exists (select relname from pg_class
where relname = 'y')
and exists (select relname from pg_class
where relname = 'z');
commit;