Do not understand high estimates of index scan vs seq scan

Hi all,

I do not understand why postgreSQL estimates an index scan only half as fast as a seq scan:

system=# explain select * from queuelog;                            QUERY PLAN
-------------------------------------------------------------------
 Seq Scan on queuelog  (cost=0.00..20530.29 rows=610929 width=148)
(1 row)

system=# explain select * from queuelog where queuelog.start_time >= '2013-05-20 8:30' and queuelog.start_time <=
'2013-06-2117:0'; 
                                                                        QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using queuelog_start_time on queuelog  (cost=0.00..13393.18 rows=316090 width=148)
   Index Cond: ((start_time >= '2013-05-20 08:30:00+02'::timestamp with time zone) AND (start_time <= '2013-06-21
17:00:00+02'::timestampwith time zone)) 


Is that solely because it nees to compare each index value to a fixed date/time?
I would assume the index would be much smaller then the actual data, or is it only based on the amount of rows?


Thanks,
Antonio


PS: here's the queuelog definition:

                                       Table "public.queuelog"
      Column      |           Type           |                       Modifiers
------------------+--------------------------+-------------------------------------------------------
 id               | integer                  | not null default nextval('queuelog_id_seq'::regclass)
 created          | timestamp with time zone | not null default now()
 lastupdate       | timestamp with time zone | not null default now()
 start_time       | timestamp with time zone | not null default now()
 sessionid        | character varying(50)    | not null default ''::character varying
 call_seq         | integer                  | not null default 1
 queue            | integer                  | not null default 1
 dial             | character varying(24)    | not null default ''::character varying
 agent            | integer                  | not null default 1
 agents           | integer                  | not null default 0
 agents_logged_in | integer                  | not null default 0
 agents_avail     | integer                  | not null default 0
 queue_pos        | integer                  | not null default 1
 waittime         | numeric                  | not null default (0)::numeric
 ringtime         | numeric                  | not null default (0)::numeric
 talktime         | numeric                  | not null default (0)::numeric
 cause            | integer                  | not null default 16
 from_function    | character varying(24)    |
 from_lookupid    | integer                  | not null default 1
 to_function      | character varying(24)    |
 to_lookupid      | integer                  | not null default 1
 maxcallers       | integer                  | not null default 0
Indexes:
    "queuelog_pkey" PRIMARY KEY, btree (id)
    "queuelog_start_time" btree (start_time)