why index is not working in < operation? - Mailing list pgsql-performance

I have a table.

\d email_track
Table "public.email_track"
 Column |  Type   |     Modifiers     
--------+---------+--------------------
 crmid  | integer | not null default 0
 mailid | integer | not null default 0
 count  | integer |
Indexes:
    "email_track_pkey" PRIMARY KEY, btree (crmid, mailid) CLUSTER
    "email_track_count_idx" btree (count)


explain analyze select * from email_track where count > 10 ;
                                                                     QUERY PLAN                                                                    
----------------------------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on email_track  (cost=12.79..518.05 rows=1941 width=12) (actual time=0.430..3.047 rows=1743 loops=1)
   Recheck Cond: (count > 10)
   ->  Bitmap Index Scan on email_track_count_idx  (cost=0.00..12.79 rows=1941 width=0) (actual time=0.330..0.330 rows=1743 loops=1)
         Index Cond: (count > 10)
 Total runtime: 4.702 ms
(5 rows)

explain analyze select * from email_track where count < 10000 ;
                                                            QUERY PLAN                                                           
----------------------------------------------------------------------------------------------------------------------------------
 Seq Scan on email_track  (cost=0.00..1591.65 rows=88851 width=12) (actual time=0.011..118.499 rows=88852 loops=1)
   Filter: (count < 10000)
 Total runtime: 201.206 ms
(3 rows)

I don't know why index scan is not working for count < 10000 operation.
Any idea please.