BUG #9553: why bitmap index scan startup_cost=0? it's a bug?

The following bug has been logged on the website:

Bug reference:      9553
Logged by:          digoal.zhou
Email address:      digoal@126.com
PostgreSQL version: 9.3.3
Operating system:   CentOS 6.4 x64
Description:

Bitmap index scan need sort the indexenrty by heap_page_id first, then out
put the sorted heappageid to next node, but why bitmap index scan node's
startup_cost=0?
digoal=# explain (analyze,verbose,buffers,costs,timing) select id from t11
where id<10000;
                                                       QUERY PLAN


-------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on public.t11  (cost=93.95..354.20 rows=10020 width=4)
(actual time=1.447..3.907 rows=9999 loops=1)
   Output: id
   Recheck Cond: (t11.id < 10000)
   Buffers: shared hit=22
   ->  Bitmap Index Scan on t11_pkey  (cost=0.00..91.44 rows=10020 width=0)
(actual time=1.370..1.370 rows=9999 loops=1)
         Index Cond: (t11.id < 10000)
         Buffers: shared hit=8
 Total runtime: 4.931 ms
(8 rows)