Postgres' join-based UPDATE updates just once instead of multiple times

test_case

/* Item names */
create table item (i_id int, i_name text);

/* Stock counts of items */
create table stock (s_i_id int, s_quantity int);

/* All rows here represent one order; each row is its line items */
create table order_line (ol_i_id int, ol_quantity int);

insert into item values(1, 'item 1');
insert into item values(2, 'item 2');
insert into item values(3, 'item 3');

insert into stock values(1, 100);
insert into stock values(2, 100);
insert into stock values(3, 100);

insert into order_line values(1, 3);
insert into order_line values(2, 4);
insert into order_line values(3, 5);
insert into order_line values(2, 6);

select
	ol.ol_i_id,
	ol.ol_quantity,
	i.I_NAME,
	s.S_QUANTITY as s_quantity
from stock as s,
		order_line as ol,
		item as i
where	s.S_I_ID	= ol.ol_i_id
	and	i.I_ID		= ol.ol_i_id;

 ol_i_id | ol_quantity | i_name | s_quantity 
---------+-------------+--------+------------
       1 |           3 | item 1 |        100
       2 |           4 | item 2 |        100
       2 |           6 | item 2 |        100
       3 |           5 | item 3 |        100
(4 rows)

update stock as s
	set	S_QUANTITY =	case
						when s.S_QUANTITY >= ol.ol_quantity + 10 then
							s.S_QUANTITY - ol.ol_quantity
						else
							s.S_QUANTITY - ol.ol_quantity + 91
						end
from	order_line as ol,
		item as i
where	s.S_I_ID	= ol.ol_i_id
	and	i.I_ID		= ol.ol_i_id
returning
		ol.ol_i_id,
		ol.ol_quantity,
		i.I_NAME,
		s.S_QUANTITY as s_quantity;

-- I expect 4 rows in result, like above.

 ol_i_id | ol_quantity | i_name | s_quantity 
---------+-------------+--------+------------
       1 |           3 | item 1 |         97
       2 |           4 | item 2 |         96
       3 |           5 | item 3 |         95
(3 rows)

select * from stock;

-- After the above update, I expect Item 2's quantity to be 90 (== 100 - 4 - 6)
 s_i_id | s_quantity 
--------+------------
      1 |         97
      2 |         96
      3 |         95
(3 rows)