how does the planer to estimate row when i use order by and group by

hi ,everybody

why does the planer estimate 200 rows when i use order by and group by .
evn:postgresql 8.4 and 9.3

table:
CREATE TABLE a
(
id serial NOT NULL,
name character varying(20),
modifytime timestamp without time zone,
CONSTRAINT a_pk PRIMARY KEY (id)
)

SQL:
explain analyze
select * from
( select id from a order by id ) d
group by id;

Query plan:
"Group (cost=0.15..66.42 rows=200 width=4) (actual time=0.008..0.008
rows=0 loops=1)"
" -> Index Only Scan using a_pk on a (cost=0.15..56.30 rows=810 width=4)
(actual time=0.006..0.006 rows=0 loops=1)"
" Heap Fetches: 0"
"Total runtime: 0.046 ms"

Can anybody suggest something or explain this behavior?