How does PostgreSQL serializable transaction works? (SELECT/UPDATE vs INSERT conflict)

Transaction 1 operated on set of data (`WHERE` clause) on which 2nd
transaction do an `INSERT`, which fit to clause from 1st transaction.
Shouldn't 1st transaction fail if 2nd commit first?

I have following table (in PostgreSQL 9.5 db)

`CREATE TABLE public.foo (id serial PRIMARY KEY, mynum integer);`

and following data

id | mynum
----+-------
1 | 10
2 | 10
3 | 10
4 | 10
(4 rows)

I run 2 serialize transactions in parallel (2 `psql` consoles):

-- both transactions
mydb=# begin;
BEGIN
mydb=# set transaction isolation level serializable;
SET

-- tx1
mydb=# select * from foo where mynum < 100;
id | mynum
----+-------
1 | 10
2 | 10
3 | 10
4 | 10
(4 rows)
--tx1: Shouldn't freeze data visible for tx1 select?

--tx2
mydb=# insert into foo (mynum) values (10);
INSERT 0 1
-- tx2 will insert next row with id 5 in foo table
-- Shouldn't insert of tx2 broke data snapshot visible for tx1?

--tx1
mydb=# update foo set mynum = 20 where id < 100;
UPDATE 4
-- Shouldn't here appear serialization fail or at least on tx1 commit?

--tx2
mydb=# commit;
COMMIT

--tx1
mydb=# commit;
COMMIT
-- tx1 Commit is OK - no any error

-- implicit tx
mydb=# select * from foo;
id | mynum
----+-------
1 | 20
2 | 20
3 | 20
4 | 20
5 | 10
(4 rows)

I am wondering why it behave so, taking in consideration PostgreSQL
documentation

> "To guarantee true serializability PostgreSQL uses predicate locking,
> which means that it keeps locks which allow it to determine when a
> write would have had an impact on the result of a previous read from a
> concurrent transaction, had it run first."
link: http://www.postgresql.org/docs/current/static/transaction-iso.html