BUG #9552: IndexOnlyScan startup_cost>0, why not equal 0?it's a bug?

The following bug has been logged on the website:

Bug reference: 9552
Logged by: digoal.zhou
Email address: digoal(at)126(dot)com
PostgreSQL version: 9.3.3
Operating system: CentOS 6.4 x64
Description:

I belive index only scan can return tuple direct, it's not need to scan
heappage, why it's startup_cost equal to index scan?
I'ts a bug?

The TEST below:
digoal=# create table t11(id int primary key, info text);
CREATE TABLE
digoal=# insert into t11 select generate_series(1,100000),'test';
INSERT 0 100000
digoal=# explain select * from t11 where id=1;
QUERY PLAN
--------------------------------------------------------------------
Index Scan using t11_pkey on t11 (cost=0.29..4.31 rows=1 width=9)
Index Cond: (id = 1)
(2 rows)

digoal=# explain select id from t11 where id=1;
QUERY PLAN
-------------------------------------------------------------------------
Index Only Scan using t11_pkey on t11 (cost=0.29..4.31 rows=1 width=4)
Index Cond: (id = 1)
(2 rows)